The upside down cat will hide until it is large enough to basically swallow neons whole. They are best kept with even tempered cichlids. Sad, because theyre awesome looking.
Maybe a 29 gallon would be better fit to your wants/needs from a community tank. That would give you room for larger shoals of tetra and corydoras. Not to mention it will be a heck of a lot more stable, parameter wise.
Are any smaller breeds of pleco available in your area? Maybe some otocinclus? Or even some shrimp to pick at the stuff may be wiser.