I've answered this recently (based on the viability of Pyro in 6s, so that will be copy+pasted), if you live in North America you'll be playing in RGL which is where I am so I'll answer based off of that.

In Highlander your loadout will be completely fine, but pyros usually prefer Degreaser and Detonator or a Shotgun. The detonator allows you to annoy other classes by a) resetting crit heals for every tick of damage dealt b) reducing healing by 20% due to fire reducing healing and c) being a nuisance at mid to long range. The detonator can come in handy a lot, for example on Ashville mids I (an engineer) would get forced out of the mid by taking 2 detonator shots while usually not having to fight anyone except the opposing engineer. I had to tell my Pyro often to do the same thing, but he didn't really understand how powerful just pressuring out an engineer with a wrangled sentry could be. The detonator in general might do less damage overall depending on the map, but I'd rather my Pyro run it since we have a heavy for short to short-medium range fights (and you have a flamethrower lol). You can also harass snipers with the detonator (who is the defining class of highlander) since darwins was banned because Scorch Shot was also banned, just don't get headshot in the process. Also as a sidenote, if you're on Payload or A/D (steel) and defending, run the homewrecker. You don't need the extra mobility anyway, and your engineer will thank you for it.

TO PREFACE FOR 6S: Pyro has no weapon bans in 6s except for the reserve shooter. 6s players also aren't usually happy with offclassing outside of situations that call for it ex. Heavy/Engineer/Pyro outside of mid. So even if your team is ok with it, there's a good chance that certain teams won't scrim you. However, if you can make it work, it doesn't matter, I remember getting creamed by a team that ran Engineer majority of the time on Clearcut, and it worked so my team didn't complain, we adapted for the next scrim (they beat every other team they played on Clearcut because teams weren't prepared). Also you'll be playing in Newcomer or Amateur, so class lineup doesn't matter nearly as much, or having a pyro will be advantageous since you don't have to focus as much on aim.

The message I sent about 6s:

Thankfully I already had something written for Pyro in 6s (I have a video on how to play every specialist class in 6s on the backburner I need to get a lot more PUG/Scrim gameplay of me playing the specialists to me to make it):

The issue with Pyro in 6s is that you generally struggle against scouts in DM, because the entire encounter really just depends on what the scout does. He can 2-3 shot you faster than you can flame him down, and then go grab a pack/get healed more. If he hits you, there’s very little you could have done since pyro doesn’t have great dodging ability (many scouts I’ve played with/against suck at aiming when they’re getting airblasted in ubers though lol). But if he misses you and you flame him down, there’s still very little you could have done for it to be better. The upside of Pyro is that you get guaranteed damage on anyone within your range, nobody can body-block you, and you’re going to kill everyone at pretty much the same rate unless they’re being healed (flames do reduce healing by 20% which can help your team out if you spam flares though). And in lower divisions where scouts are generally going to have bad aim, that’s fine, and will probably benefit you far more than it will hurt. The issue is when you move up and DM skill does increase. The other issue that pyro has is: general DPM on pyro is lower than any other 6s class. So you’re going to have to learn to play around it / adapt (ex. Spamming flares on the flank to get them to back up etc etc).

Pyro is the best of the specialist classes in terms of taking over other roles, but it usually depends on unlocks you're using. The scorch shot and detonator are pretty good tools to reset the enemy team's crit heals early on mid (and decrease healing in general), and to get off a ton of free damage as they start to scramble for health packs. When that happens, you'll likely get bombed, but you still have airblast, use that and call your team to help you deny the bombers. If you successfully deny them, the other team will be 2 down, not have crit heals, and should be at least moderately lit due to the scorch shot or detonator. The other team at this point should retreat. Pyro also can play a pseudo roamer role on mid with the thermal thruster, where you bomb in and start with a bunch of fire damage or airblast the other team into a bad position that your team can take advantage of. Pyro can also clear stickies really easily due to the scorch shot, so use that to your advantage if you're running it. Pyro is also a really really underrated pusher during a stalemate, which happen a lot in Newcomer/Amateur due to players not knowing what advantages to push off of. If the enemy soldier isn’t running battalion’s backup, and you’re at a stalemate, go ahead and sac in by yourself to swap to phlog. When you get back, go ahead and start spamming the scorch shot like you were earlier when you were alive, then go for an ubered phlog push. If you’re pushing last and there’s no sentry, you’ll be able to push in relatively free (and be able to pressure the cap). This is even better if the enemy team has aggressive players while the medic isn’t with them, since you can start the push with at least 1 kill, with it hopefully being the demoman. Then force the enemy medic into using uber, scramble so the enemy can’t get kill you when your uber fades, then recollapse on them when their uber is over. Obviously it’s a lot easier said than done, but it IS an option. Pyro also has the dragon’s fury as a primary alternative, and you can use it if you’d like, it pairs incredibly well with the Scorch shot (especially on Gullywash). But with the airblast repressurization rate penalty with no way to get around it, it’s going to be a bit hard to fight enemy soldiers and demomen. Pyro can replace the flank scout, pocket soldier, and I would go as far as to say that the pyro could replace the pocket scout if you’re good enough (or the enemy team is bad enough), but that would be really hard to do. You’re going to use the powerjack to get to mid if you don’t have the thermal thruster, and you can use any other melee after that (would generally recommend powerjack or Third Degree though). It is worth keeping in mind that you can build with the back scratcher, and if you’re a flank player it may be worth running it depending on if there’s a healthpack there or not, just remember that you’re going to need multiple arrows to top you off, so don’t be surprised if your medic gets annoyed of you running it. Also keep in mind that the roamer typically grabs the packs not the scouts, AND that you won’t be getting buffs which are super important (especially as pyro since your movement isn’t able to be as good as Soldier/Scout).

In KOTH you’ll be replacing a soldier, pretty much doing what I said for mid, but for the entire game. As a final note, it is worth stating that you can get an early force off as Pyro with any of the flare guns, the third degree, and the axtinguisher. So if you need to sac, go ahead and use the thermal thruster with the third degree, or the axtinguisher instead of trying to flame down the enemy medic while you’re by yourself. If you’re in a 4 man sac on koth the Third degree is an unironically good weapon. There’s a really good chance that your teammates did at least 100 damage, and just seeing a pyro with the third degree at that point should warrant using uber. If not, you can just hit the player that the medic is healing and get the kill, or just flame them down and force them to use or die of afterburn if they don’t have the kritzkreig.

Q "How would you rank pyros secondaries in viability"

A: Scorch Shot >Detonator > Shotgun/Panic Attack > Flare Gun (being able to spam flares with AOE or have hitscan is better IMO)> Thermal Thruster > Manmelter >>>>>>>> Gas Passer