2 pictus 3" (no longer)
3 channel cats 5-6" (no longer)
2 FL Gar 8"
1 RTC 8"
1 Albino Asian RTC 7"
1 pleco 4"
1 striped raph cat 1"
1 shovel nose 8"
110g 4ft tank
Did I get that correct? I don't think I missed anything. You're incredible overstocked which will attribute to the very high nitrates. With a stock like this you won't be able to keep the nitrates at a safe level in the tank, period. Plants or not, extra filtration, w/e. You need to seriously reduce your stock. Not to mention one of the two RTC will likely start to reduce your stock for you in the near future assuming they survive the situation you're currently experiencing in your tank.
The aerator being added to the tank, as a few have mentioned, will help add oxygen to the water. The agitation that is created by the bubbles popping at the surface of the water pulls oxygen down into the water. That can't hurt.
As far as some fish dying and some not, some were just able to withstand the poor conditions and some couldn't. Like others have already mentioned some fish are just hardier than others.
Not sure if that ammonia reading was before or after your wc, but if you are reading any trace of ammonia at all it's unhealthy for your fish. That also wouldn't surprise me in your scenario since you are so incredibly overstocked your filtration might not be able to handle the amount of waste in the water volume hens slight ammonia readings and high nitrate readings.
Currently your main issue is the nitrates. Only way to correct this is water changes. I'd keep up with them quite frequently with the stock you currently have in there and try to keep the nitrates below 20ppm and ideally around 10ppm since your fish might already be in poor shape from lack of oxygen and exposure to high levels of nitrates, and possible ammonia.
In summary, I'd keep up with the water changes to get your water conditions at a healthy level and keep them there, added aeration certainly can't hurt, and seriously reduce your stock to something much more reasonable for a 110g 4ft tank. Good luck with your tank.