HMB is short for beta-hydroxy beta-methylbutyrate. It's a metabolite of leucine. It's soposed to have an anticatabolic effect on the body and reduces protien breakdown during excersize. It's also been toted to help with DOMS (delayed onset muscle soreness). It would be fine to stack with ZMA. My opinion on the stuff is its not worth the $$, ive used it myself and noticed little to no difference while useing it.
sounds like a good stack me! i would throw some beta-alanine in there...good strength and endurance increase, without bloat or water issues. Theres lots of research on the stuff, its cheap, and it gives a great pump.