Yeah, that's what I mean - most of the standard gretchin box is grots with one or two guns and no arm choice, so to distinguish your counts-as bomb squigs from normal grots, it's preferable to do something to remove those guns. The pistol-grip hands limits your options, unless you want to replace the whole hands too.
But if your opponents won't care, the easy option is just to load up standard grots with a load of stikkbombs and claim that the guns aren't loaded or are broken. Pedantically, there's little in-game confusion really, unless you're likely to have a grot squad mingling with a tankbusta squad.
The other option is
WHFB goblins, especially those with spears - a spear shaft is relatively easy to convert into the stikk of another stikkbomb