First off, welcome to Dakka
Second off, how evil do you want to be?
-Due to the opponents you describe, a drop list is almost a hard counter to them. Plus, because it's a FFA match it'll save you quite a bit of shooting that you can be the victim of.
I'd run something along the lines of:
-2x Captain
-2x Sternguard squad
-2x Tactical squad
I would tend towards a symmetrical force in a free for all match. That way you split your army exactly in half to take out one side, then use the other half (that can potentially come in together) to either finish off one side or sandwich the other.