Once you plug a cable into the Mono out of the F2B, there will be some cross-talk between the A and B outputs. You will have to decide if it's problematic for your situation. The times I've used all 3 outputs, it has not been an issue.
The level should not be too hot out of the F2B, but will depend on the volume setting - since you will be sending a post-EQ signal.
If you have rack mount DS-5, you can use both the front and rear outputs at the same time. I have done this in the past in order to send a pre-EQ signal to the FOH and feed an F2B on stage.