Crossman BBs are .12g and as such as utterly useless in anything but shitty springer pistols (which happens to be what Crossman softair pistols are). You CAN however use them (and in some instances I would recommend it) in multishot shotgun cartridges (Maruzen shot shells) and 40mm grenades; heavier BBs group too tightly, for one, and the limited gas pressure won't send large volumes of BBs very far. So the lighter, wildly innacurate .12g BBs are actually desireable here to get a bit of extra range and a larger shot pattern.
BB-ejecting handgrenades, AP mines, etc can also use from them (they have no barrel, you just want "stuff" to fly out).
As for the plastic slide Q., it depends on many factors. SOME guns, specially the Taiwanese ones, are designed to shoot propane (despite having plastic slides). In fact some of them will not cycle properly on weaker gas. It's not just the quality of the plastic, there's physics involved here; those Taiwanese guns have much stiffer recoil springs, which absorbs the heavier blowback just fine.
Similarly, Japanese guns are designed to operate on lower pressure gas, but upgrade kits are available (stiffer springs etc) that make it possible to use green gas/propane in them.
And just to mess with you a bit more, metal slides CAN also crack; some (especially older ones) are made of pot metal, and aren't any better than plastic when subjected to heavier blowback with weak recoil springs (a perfect example is the old Zeke metal slide for TM Tac Master - don't know if newer models have addressed this).
So if you aren't getting a gun designed for green gas (propane), either stick to blue gas (134a) or make sure you perform your upgrades smartly. (by way of reference, I have a WA 1911 that I shoot Green with, which still has a plastic slide b/c i can't find an adequate metal one for what I want to do, and it's held up fine with an upgraded recoil spring.)