It'm guessing it's a 10 gallon tank, right? I'm going on the asumption that it is.
If it's not, and it's bigger, then you may want to upgrade the filtration a bit...especially with a Pleco in the tank, as they are super messy fish. And if it's the 2.5 gallon thats in your profile, then really the only thing sutible for a tank that size a Betta.
I used to keep only three Danios, but after adding two more to make a school of five, I've found they are much better behaved...not to mention they get along great with my boisterous Tetras.
Neons are super fragile and should only be introduced to a very mature tank...Cardinals are a bit hardier, but should still be held off on for a while.
I don't know about the Butterfly, but I think they may get a bit big for a 10 gallon...not to mention I believe they eat anything that will fit in their mouths, so I'd have to say the Neons (and posssibly the Danios if it can catch them) are out of question.
And yeah, the Plec is gonna get big. If you're not thinking of upgrading the tank soon, I'd consider taking it back and getting something a bit more to suit the size of the tank. Oto cats are a great choice for a small tank. But if you want to stick with Plecos, I'd say try a Bristlenose...great little fish they are, and they generally only get to about 6 inches max depending on the species. You just have to be sure you don't overstock if you add one...they are very messy fish, like most Plecos.
Hope this helps!