There should be no problem as long as the MSC is properly configured 🙂 you are right, the only tricky parts are
– the HO BSC-BSC
– the Idle Reselection BSC-BSC
– the PS Reselection BSC-BSC
but again, if the MSC is well-configured, then there should be no problem.
For the PS reselection, it is up to the MS to reestablish the TBF in the new cell: I don’t think the SGSN has to play any part in this.
And yes, this configuration is deployed around the world (mixed BSS), without any issue, except MSC misconf. That is the whole purpose of a standardized 3GPP GSM: it intends to work across several vendors.