Motion 5 project files are not compatible with FCP 7.
No clue about Compressor 4.
Working with FCP X is totally different, totally new, and faster and easier than 7, once you learn it.
I'd not bother trying to pull transitions over from 7 to X. It is going to be a ton of PITA work, if they even work. Simply building them from scratch with Motion 5 is very easy. Thrid party plugins for X are way cheaper than they were for 7.
If you do want to pull "transitions" form 7 to X, do it in Motion 5. Start with a "Final Cut Transition" template to pull all those QT files into and work from there.
IMHO, plugins are not a reason to not move forward.