In theory, yes parallel packages libmirclient8 and libmirclient9 are "compatible" in that they don't conflict with each other and either will work for those packages linked to them. Both should speak the same protocol to the same server in the end.
In practice Mir had a rare protocol compatibility break (LP: #1486496) in Mir 0.15 that might break the older packages ability to function correctly. Not sure.
In theory, yes parallel packages libmirclient8 and libmirclient9 are "compatible" in that they don't conflict with each other and either will work for those packages linked to them. Both should speak the same protocol to the same server in the end.
In practice Mir had a rare protocol compatibility break (LP: #1486496) in Mir 0.15 that might break the older packages ability to function correctly. Not sure.