I have some feather boards that have aluminum tracks that go into the miter slots and expand out to hold the feather boards in place. I haven't used them yet, however. I did try them in the slots to be sure they work.
Hard to say what the issue was. I would check the alignment of the blade to the rip fence to be sure they are parallel. If that is not the issue, then maybe the boards were not straight. I have a joiner but it is an old Inca and relatively short. My skill level is not so great with it so I usually use my track saw and long guide to get a straight edge. But some wood moves again when I rip to width. But usually that does not happen. With a correctly tuned saw and a straight edge on the board, I don't have a problem putting a rabbet in the edge. I almost always use my table saw for this. I do not love screaming routers (but I have 4 good ones). It is safer to put a sacrificial surface on the rip fence and cut the rabbet on the rip fence side of the board. You cannot go too deep that way. I usually don't do this but if I was having trouble I would. I usually only do this with a dado blade.