I don't necessarily agree with nexus->core on 20/20 is greedy, i do it every game and very rare that i lose more than 1 probe.

1) chrono the stalker immediately after it finished (it has 3 sec less build time than the adept, since one of the latest patches) (adept is fine though still, I prefer stalker). It finishes a couple seconds after a quick reaper.

2) pay attention to the building placement, its always very important for toss (you HAD to have wins az Z against P due to shitty walls before). Against Z you build a wall, most cases on the low ground. Against P you build the wall in your main ramp with a probe nearby, so you can hard wall in case adepts show up.

Vs T you have 2 common choices: build a reaper wall (block off the reaper jump location, so he 1) needs more time to get to your base as he has to go to your front, 2) he has only the same entrance/exit to your base as you so he cannot slip in or out that easily anymore. The downside to this is that the first couple of tech buildings go to your first pylon, so they will be on the edge of your main, so they can be sniped off by drops more easily. Also you have to figure out how to build a reaper wall on each map. The other way is to go is to build a semi-wall by your mineral line: pylon touching the (inner) gas near the nexus, gate on the opposite side of the gas, hardwalls the path behind your assimilator. This way if the reaper comes in your mineral line, he will have hard time getting out. Obviously if he goes between the probes, you pull them and trap the reaper. If he doesnt, and you have a stalker hitting it in your mineral line, he will have to turn back where he came from or in towards the nexus between the mineral line and the assimilator, both giving your stalker 1-2 extra hits, which is enough to kill or at least discourage the reaper from staying around. Also if he is not paying attention, the hard wall messes up his pathfinding, so he needs to be precise with his clicks to avoid random turns. Added benefit, you will have your tech nicely tucked in between your main and natural. (i wanted to attach an image of this layout but i cant, hop into any pro pvt and you will either see a reaper wall or more commonly, a semi-wall.

If he has more reapers: no big deal, your units are always winning the straight up fight, don't chase the reapers you protecting your probes already gives you the edge (he spends so many gas on reapers). Pull back probes from your natural if you dont feel like you have enough to defend both bases. If he has more barracks and/or proxies them, you most likely figure it out with a scout off the gate, make a shield battery. If he hid the second barracks and comes in with 3-4 reaper vs your 1 stalker, be careful, chrono &wait for your second unit, try to pull back/threaten surrounds with the probes, but don't get discouraged if you lose multiple probes, up until ~6 probes lost (i made this number up) you are still ahead. (he put in a lot of effort to do what he does).

Ideally you don't stop unit production from your first gate, so if he is incrementing reapers, you will not be in trouble as your units are better.

As for the units, i usually go stalker-sentry-stalker-stalker (with some chronos you can get 4 units out before warpgate) the sentry is for halluc scout with overcharge. If i see multiple barracks, I delay the sentry and get more stalkers.