He's only using a 14th round pick for Andrew Luck so there's no reason not to draft another starting quality passer. It just means he doesn't have to break his draft to get a safe one ASAP. I think Luck a great keeper choice with excellent long term potential as long as you don't reach for him - which he didn't.
Because I wouldn't touch Ryan Matthews with anything more than a throwaway pick. His yards per carry and goal line efficiency are both awful and he also is constantly nicked up with nagging injuries. The new Chargers management realizes he's a colossal bust and are certain to draft at least one runner, whether they get someone in free agency or not.
I'd go with BJGE for an 8th round pick as my 5th keeper. They'll probably bring in a change of pace guy, but the Law Firm should still end up with most of the first and second down plus short yardage work. At worst (barring injury) he'll be a spot starter and you could do much worse with an 8th round pick.
One question about the league format. Can you conveivably keep players forever, or is there only a set amount of time before someone must go back into the draft pool?