Corys are bottom feeders, and with such a small tank there isn't enough floor space to house all those corys.
Neons generally need 20gals as they're quite active fish, I'd say the same for guppies too.
Also your stocking level can be affected by what filtration you have, if you are over filtering (I recommend this in the highest!)
But I'd say with the guppies and the cory group you wanna add you'd be overstocked, but safely overstocked.
A 50-70% pwc
will keep the tank in healthy condition.