what kind of fish is this

I believe it's some sort of African Cichlid mix. they're usually reffered to as assorted cichlids. Don't know for sure but that's what it looks like to me.

I would also say it's a Cichlid of some sort, though I am not sure of its name. I have seen them at my LFS, however.

It would be a good idea to post this on the cichlid forum, someone there can probably tell you for sure, but I'm going to hazard a guess.
This is certainly a cichlid. It looks like a lake Malawi cichlid, maybe Haplochromis sp., but I'm not an expert; I'm looking at Dr. Axelrod's Atlas of Freshwater Fishes and that's what it looks like to me.
If I'm right and it is a Malawi cichlid, it's not compatable with the fish you have in the tank right now; it needs a pH of about 8.2 and harder water than the others.
Goldfish and tropicals aren't a good mix, they need completely different types of conditions. The angel, tetras and red tailed sharks will probably be fine together, they need similar conditions, and a 35-40g tank is large enough for them.
Take the unidentified cichlid and the goldfish back to the fish store, they're just not suited to life with the others.

Looks like Haplochromis electra to me. I guess it's in the genus placidochromis now. African cichlid from lake Malawi.

PH of a fish's natural water is only necessary if it is wild caught or a generation or two after that. By the third captive held generation you can forget about it's "natural PH". Fish are very adaptable in that way.

the natural conditions are more important if you have eggs or wigglers but the adults are alot more adaptable