I might add some pygmy corys to the tank, but how hardy are they? I watch my perams pretty well, my tank wont be cycled but its 20g long with a single male betta and 3 nerites. Im planning to add 4 pygmy corys and that will be the entire stock so I dont imagine the bioload will really be all that bad, especially since my hormwort does a good job helping me with ammonia. My Ph reads 8.0-8.2...betta and snail are perfectly fine with it...but I dont know about the pygmy corys, I read some info on them, some sites say no higher than 7ph and others say 8 ph is fine...
You should be fine in the 8.0 area, but they probably won't breed for you. It sounds like you want a community tank, and breeding isn't a concern. If they did breed, it's unlikely that the eggs would get fertilised. Lower PH softens the egg membrane for fertilisation and hatching.
One other note: pygmies love java moss. I can't tell if those are live plants or plastic in your pic. Java moss would definetely make them happy.
It'll be fine, your 20 gallon has more than enough room for all of them. One suggestion is to add an airstone. My corys go crazy for it, swimming up and down in the current. Its absolutely the cutest thing!