Yes, I have had that awful experience of having to get rid of partitions and it is a headache. The advantage in this case is that the amount of data on the old computer's hard drive is a known quantity so I can make that partition to fit (The old computer has a small internal drive.) Then the rest is TM. BTW, I think I can make the partition that has their old computer's data 'read only' and then there won't be any problems with any files on that getting deleted by mistake.
Don't forget to leave some extra "overhead" space on the old drive partition. Space that would be used during the processes of copying, moving, etc.
