Kotobukiya MSG weapons do work with 30MM/MS but the grip doesn't fit the hand snugly so you have to balance the weapon carefully. 30MS hands don't open up so you'll need to squeeze the grip into it. The hands are somewhat elastic so if you're careful, it can be done. (Last two pictures)
MSG weapons also have various 3mm holes so it is very easy to use those holes to connect them into your 30MM/MS kits.