Fish Tankmates and new fish?

I have a Golden Sucking Loach (Algae eater), a Red-tailed Botia, a Red-tailed Black Shark and two Firemouth Cichlids. the Sucking loach is the largest of them (he's about 16cm long where as my other fish are around 9-14cm.) and he keeps on chasing the other fish, not so ofton now though since the others have grown a bit. Anyways I'm getting a Channa Orientalis (Ceylon Snakehead) since I've ordered it from a local fish store, I've been waiting for it for about three months! (I'm from England so it's legal up there.) The Women said that they haven't found one yet so they may hybernate or something. Would the Snakehead be okay with my fish? How big do they grow? (my tank is 100 gallons.) The thing is I brought one from a fish store and.. I had it for about a day and then next morning it wasn't in the tank! I lifted some clothes off my floor and it was litrally underneath them on the floor! (it wasn't the same store I ordered my new one from.) It was covered in fluff, probably due to my carpet. we quickly put it back in the tank and after a while it started swimming around again but then it stopped moving again and it died. We didn't know it was an escape artist so we've put sellotape to cover a small hole what i've got on my tank. Oh and almost as soon as we got it it went up onto the pump. I don't why it did this but my Dad reckons it's because it needs air every once in a while in other words it needs both water and air to help it to survive like frogs. Please help.