heh, a heavy stubber? I'm not quite sure how you thought you could get away with that.
Yup you can give a scavvy a plasma pistol. The rules get a bit tricky because the Scavvy rules use non-standard weapon category names. Basically, scavvies can be armed with any of the Pistols and Hand-to-Hand Weapons listed in the trading post rules. However, all Pistols and Hand-to-Hand Weapons not listed in the Scavvy weapon list under the Ranged Weapons and Close Combat Weapon categories follow the usual house weapon list rules (i.e. you must expend a rare roll to buy it). So to buy your mutant something like a chainsword, plasma pistol or lasgun, you must expend a rare roll during the post game sequence to buy one of those weapons.
Is that any clearer?