I've always used 31g slin pins (1/2") for IM. Any slin pin will work for for bis, tris, delts, pecs, quads, lats. You should be fine unless you have a higher BF%.
Yeah, you probably would need a 1" pin for glutes. I don't know why you'd want to shoot glutes with IGF. The idea of going IM is for some site-specific growth. I'd rather have any site specific effects work on a visible muscle than my butt cheeks.