so at my lfs they have what they call a belsonsox gar ( sp) but my friend told me it was realy a killi and realy going to get rather big it kind of looked like the yellow Panchax Killifish but alot more plan its brown but hangs in the same area and has the same shape so heres my questions is it a killi can i keep it in wih my eel and corrys and butterflyfish and how big will it get ( sould i jsut want a pair in there own tank)

Belonesox belizanus is a live bearer from Central America (I think). grows to about 8inches, comes from hard alkaline water with a bit of salt. And is an ambush predator. They hide under leaves and wait for something to swim by. Then they snap it up.
Anything that doesn't fit in its mouth should be fine but they have a big mouth.

yeah but going in with a eel a black ghost knife bigger then he 3 cories and a bunch of butterfly fish seems like hes my oddball

You never mention how big is your tank. Black ghost knifefish probably will need 150 to 225gal tank as they can reach more than 20 inch long. How many butterfly fish?

i have 4 butterflys and they are in a 55 right now becouse the bgk are relitivly slow growing im moveing them into a 90 by fall or so ( the bkg is the second biggest in my tank at around 4 inches i also have a 6 inch peacock eel but he doenst bother anything he only seems ot eat live worms weather there tubflex or blood or glass or earth