Online compatibility comes down to the host disabling/enabling certain aircraft in the corresponding multiplayer menu. So if you don't want to fly FP planes online just dissble them.
FP also modifies a number of stock gun weapon files which could cause multiplayer missmatches but it comes with a file swapper to take care of the issue.

As for the quality, well it's a very mixed bag.
On the plus side, it comes with a lot of aircraft.
The quality of the crafts varies greatly and goes from swan to ugly duck.
If you are into historical accuracy, and like me with a particular interest in Luftwaffe crafts, just forget it.
It has a lot of Luftwaffe crates but research has been done pretty poorly with phantasy paintjobs, nonsense designations and loadouts and what else.
The missions you can fly from German site are not even close to have any historical evidence. Again, the creators obviously didn't do their homework.
Can't say much about the allied missions as I never flew them.
All in all good return for the money as far as sheer number of planes goes, just don't expect it to be, err, did I mention historicall accuracy?