Depends on what you're doing. But in general, the extended communities and the RD will usually have to be compatible, and the address-scheme may need to be compatible also.
But the real question is - why do you care? If you're just asking out of academic curiosity, then that's cool. But I hope you're not actually thinking of implementing such a setup, because at this time there is basically zero business-rationale for doing so. Service-providers are rightfully more worried about putting their financial houses in order rather than playing around with weird bleeding edge stuff like carrier-over-carrier.
