I believe that the Companions always auto-level to your current level once they are switched back in. Henchmen do not. So level 20 is as easy as getting the Source Hunters to level 20.
The real barrier to not switching party members is not much of a barrier, it's just that switching them around could miss one or the other of some special dialogue, and you generally want to keep the ones you have to match your party's configuration.
A balanced party is generally best for the first run-through. What combination you want is up to you.
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Well, now I want to see the mod "Source Hunter's Day Off", starring Madora, Bairdotr, Wolgraff, and Jahan as the party while the Source Hunters are passed out from booze in a tavern and miss everything.