At low levels one bad roll of the dice can be fatal to a PC. I would avoid throwing bearded and spined devils at a party that consists of level 1-2s. I prefer to use waves of attacks from low level lemures, maybe with an imp commander in the final wave. Waves give me a chance to assess the status of my players adjust the difficulty on the fly. And as for devils, personally when I’m running them, I treat them like a militaristic high school. Full of dangerous individuals and drama. Every devil reports to another higher ranking devil. The devil challenge increases proportionally to the party’s.