I use 18g to draw and 23g 1.5" in glutes, 23g 1" in quads. I always get a lump if I use 1" in glutes.
Regarding 18g causing rubber from stopper, I rotate the draw location each time. I've seen a chunk of rubber missing from the top before I started rotating the location, and I don't know where it ended up... My hope is that if it was in the syringe, the 23g would not let it pass through.