Just because there is a Rigelian Collective, does not mean such a group MUST be based on Rigel. It could simply be a name.
The Battletech starmaps were never, to my knowledge, meant to be 100% real world accurate. The maps were there to give context to stompy robots.
If, for some reason, you NEED Rigel to be in the Taurian Concordat for your game, then simply put, it is.