That should work, I think. The four 2nd level minions are another EL 6, so the enemies will not be slaughtered, particularly if they use sound tactics, but the PCs still have a good advantage, so they will come out victorious unless the dice go horribly wrong (especially since the enemies don't seem to have healing, and I'll assume the PCs do--that's a big advantage in this kind of thing)

Bumping up the two main NPCs to level 5 (though admittedly a level 5 sorcerer isn't really strong enough to be a CR 5 challenge) and upping the minions to level 3 will make the encounter enough to give the PCs a true run for their money.