Can one go from 7-9 (skipping 8)?
Yes, of course. In fact, where space is tight, it could even make sense to have a system with Logic 7 for backwards compatibility, and Logic 9 as the current version with all the latest bells and whistles, and leave logic 8 out altogether.
Thats exactly what we did. (or are doing I should say) Cloning everything on one machine. That way we can slide out the hard drive sleds on one system (containing Logic 7 and Tiger) and slide in another set containing Logic 9 and Snow Leopard to test for incompatibility issues. I'm going to skip going to 8 altogether (thank god) and also skip Leopard (i.e. go straight to SL)
Its my understanding the original 9 is more stable then its update.