You can have seriously bad things happen to you if you combine two kinds of off-the-shelf medication, or if a doctor gives you a prescription without knowing that you're already getting a prescription from another doctor. Combining drugs, or taking multiple doses of a single drug, generally gives you little extra benefit, sometimes less benefit, and often drastic negative effects. That's just normal, not-even-illegal drugs - I don't want to even think about what speedballing nitro and kamikaze might do to a character. I think cardiac arrest would be likelier than a combination of the bonuses.
I never played Rifts, but didn't juicers have something implanted that regulated the doses of all those chemicals? If you house ruled such a piece of 'ware, then speedballing combat drugs becomes at least marginally plausible.
I would also allow combat drugs to stack with bonuses from other sources - I could see someone with kamikaze, an infusion of sideways, oxyrush nanites, and an adrenal gland. Still, the disadvantages are that you are getting a temporary boost compared to an augmented person or an adept who has those abilities 24/7, and that generally, there is a hell of a crash afterwards.
