If you are gluing both Guardsmen to the base (as opposed to just the gunner, or mounting them both on 25mm bases), I would assemble the gun, the gunner and the assistant/ammo loader as separate pieces. Put them on the large base to check fit, and when happy, glue the gun to the base. Paint the gun. Paint the Guardsmen separately, and mount them when done, then add basing material. In general, always glue models together, then paint. You will get a stronger bond that way, and your new troops will stay in one piece longer. My preference for gaming pieces is the strongest bond over all the details. However with the kneeling Guardsmen, and especially the large base, you may have problems painting all the nooks and crannies. Good black primer should help cover any areas you can't reach with a brush.
Good luck, and post some pics when they're done!
