I like the idea of the Federation council having a tricaramel legislature: (in order of power)
-One entirely based on population groups (basically like then House of Reps)
-One where each planet (besides colonies with tiny populations) gets one Councillor.
-One where each species gets one Councillor.
If a single family of Cardassians moves to Vulcan in the year 2267, does that mean that you have to add an entirely new Councillor for Vulcan to the House of Species?
The idea is that it would be for species which have officially joined the Federation via their homeworld, and it would be much smaller body than the Planetary one, which would be bigger due to all the colonies. I admit it's sort of a flimsy idea, and maybe it could be a committee in one of the other two or something.
Also, it wouldn't work in the way your asking. It has nothing to do with planets. So, if it were so specific, there would just be a Cardassian Councillor who represents all the Cardassians in the UFP.