Horus Lupercal have a point: in like 30 rockets that I built I used once separation motors, only because the boosters recontacted the main stage, so two solutions: or as I did you use ion engines, that can give a nice slow detachment or you bp edit a Kolibri and give him a practically empty tank to do an high trust-small time burn (I never tested the second option but with a bit of tweaks that should work.)