The mobs will spawn at level 20 on the lowest difficulty setting regardless of team level. Only the team leader's difficulty setting matter. If the team leader changes in the middle the new team leader's difficulty will be in use in the next mission he selects and starts.
I recommend getting as many players 19 and above on this Task Force as possible to start. Team Composition as it relates to powersets and archetypes is always irrelevant in my book. I'd simply try to get 4-6 people in the 19+ range with hopefully some players malefactoring down. Some of the missions are actually tougher with a full team. You will want enough to defeat an Archvillain. The archvillains are relatively weak so 4 to 6 level 19+s shouldn't have too much of a problem though.
Hey thanks man, that is a good answer, I wish I could add rep haha.
What lvl are the mobs that spawn for this SF? They have changed the way mob lvls are determined so many times I can't figure it out.
What is a good lvl to start doing this SF?
Anyone have any tips on team building for this SF?