I’ve had BM5000 and BM5500
BM5000 can only be controlled with MCP5000 or terminal 5000.
BM5500 was the first in that line-up which can be controlled with Beolink 1000, 5000, Beo4 etc.
Since BM6500 can be controlled with those remotes as well I see no reason why MCP5500/6500 shouldn’t be interchangeable.
However… there might be some Beomaster specific stuff that won’t work… timer function or similar. But generic functions shouldn’t be any issue.