Ever notice that Tiamat is pretty much a huge Fire Dragon?
I mean look at her, that's exactly what she is, the only difference is Tiamat can use Dragon Driver, which is a summoning sequence type Psynergy as with normal Fire Dragons, all they can use is Dragon Fume and Fire Breath, however I but Tiamat can too if she was given the opportunity.
My point is pretty much that Tiamat is probably of the Fire Dragon race.