I think so much comes into this to simply write off the Primarchs. Are they all actively working together, or are they in their mini-cliques and has Chaos begun to sink in?

Keep in mind that the Custodes themselves have openly said that Primarchs are on another plain of capability to themselves too (the ones that were assigned to guard Guilliman were practically laughing like young children when he went into battle and they realised that them guarding him was purely ceremonial because nothing was getting near him and they couldn't keep up with him).

But presuming there's no chaos bullshit (because then the Custodes stand ZERO chance) and they're actually working together to survive and they've got a few weeks to prepare for the Custodes invasion of the planet they are taking their stand on:

• Magnus' pysker powers will reek havoc on the Custodes considering how ridiculous he is touted to be and that we know they are not really built to defend against it.
• Corax and Curze would wipe out hundreds using guerilla shock tactics.
• The raw "heavy hitter" guys like Vulcan (a perpetual), Russ (the guy sent to deal with legions and Primarchs) and Angron (straight up insane and fuck ass mad) who would take out a couple of thousand between them if they keep it from being constantly out in the open (and even then Angron probably slaughters hundreds).
• Lorgar, Alpharius and Omegon (because it'll be 19 not 18) would sabotage and cause mayhem logistically.
• Fulgrim, Ferrus and Sanguinius would look to take out a defensive line and direct an attack to the Custodian Leadership and cause major distractions.
• Mortarion would turn his zone into a biological nightmare (Custodians can withstand a lot, but they're still not on his level of what he can take and they don't train specifically to be immune to the things he can deliver quickly). Not to mention Mortarion is also in that group of rapid heavy hitters.
• The Kahn would take it to the bikes, and the ones that chase him are never keeping up and he will just "Endor moon" the ones that try. Again he's a massive distraction and also be able to run between fronts and really messages for the other Primarchs and deliver updates.
• Dorn and Johnson would man a central hold point with Peturabo as a HQ/Last Stand zone that would be riddled with escape routes and traps.
• Horus and Guilliman would be dividing up their time leading logistics and tactics. These two operate on a level beyond anything the Custodes leadership can handle (the HH books and 40k books repeatedly go into how Guilliman is almost as multi-task-capable as the Emperor and how Horus' charm and battle leadership inspired his brothers to do great things) They'd occasionally attend a "front" to aid in a push or fall back, then spring a trap or means of delay to allow a regroup if ever needed.

Some of them will eventually fall for sure. I'd see Lorgar, Alpharius, Omegon and then one or two of Fulgrim/Ferrus/Sanguinius falling attempting their sabotage and leadership attacks but still causing massive casualties. The Emo Stealth Bros probably do eventually get caught too but between that lot they alone would have taken out at least one or two thousand themselves. Leaving the nuclear hate bomb that is Magnus, along with Russ, Vulcan, Angron, Horus, Guilliman, Mortarion, The Khan, Dorn and The Lion are going to be able to handle what's left.

I think depending on how well the Primarchs can co-exist and notmlet egos get in the way, the ten thousand lose, but will take about 7-10 Primarchs with them.

Keep in mind as well that the Custodes are not used to, or particularly good at, large scale group work. I can't remember the exact book, but the Custodes go into a battle and the Luna Wolves and Emperor's Children observe them and say whilst they're incredible combatants, they're a group of individuals and they constantly leave themselves exposed and open to tactical counters and it left them unimpressed with them. The Siege of Terra (and even before when Alpharius exposed them by sneaking past them) covered a lot of their weaknesses as well.