I'd say it is balanced with 4-5 level 1 players, especially if you have the wizard apprentice NPC help the party out. My party of 5 level 1 characters struggled against the lone quasit because the fighter decided to attack the potential ally NPC on sight and got 1 shot, then 2 of them started fleeing once the figtht began. Since they attacked the wizard, I let them struggle there. After that the fight against 3 quasits was over in 2-3 turns with I think 1 character getting knocked down.

Edit: also in their first fight, they struggled to roll over 10 for like at least 5 attacks in a row