I got a freshwater 5.5 gallon tank. It has a mini aquara filter. I got 3 guppies 2 neons and one common plec. I'm just wondering what fish could i put in with them.

Sorry to tell you but your tank is already overstocked. it is full with just the guppys.

btw common plecs grow HUGE and need at least a 50 gallon tank.

I agree with the above, that tank has way too many fish in it already! If your guppies have fry (read your other post) wat are you going to do with them? There's no way there'll be room in that tank.

Sorry mate.

my guppie just died so i got 2 guppies now

Im sure the guppies are alright but my
main concern is the pleco
they can get to be 18 inchs.
So you might want to put him into a bigger tank

The guppies will not be alright, the tank is far too small (IMO). It's no wonder one died, I wouldn't expect the others to last. How long have you had them like that?

Also, did you research your plec? They do (as Black Oranda said) get to be 12" and often more, they grow quickly too.