Is my fish tank overcrowded?

You are doing ok if that is what you want but if it were me, i would only do about 2 or three corries and have either another small school of tetras like cardinals or neons or you could get a pair of German blue rams. i tend to like cardinals they school very well and are very pretty fish and i would consider doing those with GBRs instead of black skirted. Germans are a dwarf cichlid which is semi agressive but if you have dense enough foliage or good places for them to hide your other fish will be fine. i would avoid the rainbow shark it is less aggressive than red tailed but still slightly agressive and they swim a lot and need places to hide. i three assassins in my 15 g column right now and they have done well so i wouldnt say more than 5 or 6. but in the end if you want 6 of each of those, dont add any more

4 assasins will do nicely. You got some compatible fish that will do nicely in a planted softwater setup

A pair of a small cichlid such as the ram or the krib could work if your setup allows for territorial boundaries.

i would say its ok but also recommend adding no more.

also make sure you have a great filter, the cleaner the better.

i have a small 55 gallon tank and have two 90 gallon filters and it is crystal clear.

your fine but dont add anymore