You can join RP without leaving your current corporation, too! Please refer to So you want to get into roleplay? for some ideas on how to find RP.
Caldari mercenaries and Caldari expats tend to be everywhere, and also in Minmatar RP organizations. Whether a corporation ran by a Caldari would be a good fit for a Minmatar alliance depends on things other than the ethnicity of the CEO, I think.