Since this hasn't had a lot of response I'll take a shot with my limited experience with Thorichthys. I've only owned Meeki, but have seen others with different species on here, particularly maculipinnis. From what I understand thery are relatively aggressive to similar shaped, same species fish, but for the most part mind their own business when it comes to other species.
What species in particular were you thinking about? If Meeki, get ready for a species that has relatively low aggression towards other species, but threatens a lot (flaring its gills). Meeki will be the most common, but is still good looking in its own way. Maculipinnis is similar to Meeki, looking almost exactly alike but with a few more splotches of color across its body, from what I have heard they act very similar as well. The only other species I know is Aureus, which most people consider the best looking, act similar to the others but perhaps slightly more peaceful, but require a little higher water quality than its more forgiving cousins.
The question, will they be able to live with molly's can only be answered with a maybe. It will be touch and go, watch them carefully as they will grow in aggression as they grow in size like all cichlids.
Those 3 are the most common that I know of. Google comes up with many I've never heard of, which may be unnobtainable, but they are worth a shot as well. Hopefully someone with more experience with other species in the Thorichthys tree will be able to help.
As far as the Rainbow cichlid goes, never owned them, know nothing about them. Other suggestions might include non-breeding convicts, a severum, Dwarf Ram species, perhaps even an EBJD if it is peaceful enough.
