[Filip Kalinowski]

Just did it on fcp 7.
I had issue with synchronization(propably due to lack of same timecode on all clips – I was using in-points for synchronization on first 4 clips).
To achive synchronization with rest of multiclip I needed to synchronize new clips with my clip form V1 track, than (becouse there were different timecodes and both clips had different lenght) I exported new clip as a reference file (faster export). After synchronization my new clip started exacly 00:00:33:04 later than my V1 clip, so I added 33:04 of slug at the begining of my new clip, than after inserting my new clip into multiclip everything was synchronized.

hope it helps if anyone will have issues