I would probably keep to just one species per tank and get another fire bellied newt.
Not heard of anyone keeping anything different together as I would think that problems would arise like one eating the other, one toxining out the other, fighting for territory etc.
please dont mix with anything else you may get away with it for a while, but it will almost always result in catastrophe check out this page http://www.caudata.org/cc/articles/Mixing_disasters.shtml and whatever you do, dont take any notice of pet shop owners who will tell you almost anything to make a sale