I'm not sure about the other bijuu. They're obviously powerful in their own right, so it's a fair point to bring up.

They did say the bijuu are born where hatred festers, like in Sasuke's (and I presume Madara's) heart. If the Uchiha are the revenge machines Madara is describing, the other bijuu could be the chakra manifestations of Uchiha hatred toward the Senju.

Assuming that the bijuu come from hatred chakra-ified and released as an MS technique, then combining all the bijuu could release an "ultimate MS", or even the Rinnegan, much as you describe.