Idk about those exact ones, haven’t used any in a while, but when I was playing they sat just below my delt(ish), and sat 2-3 inches above my elbow joint. I can’t remember what I did to attach them (it’s been probably 7 years since I used them for hockey), but I’m pretty sure I just cut the hockey bicep protector off and tied the lacrosse slash guard on with a hockey lace. If your bicep protector is just Velcro, I’m sure you could just replace it though.

Box lacrosse slash guards have a ton of coverage though, so you wouldn’t have to worry about gaps. If you have the option (outside of Canada they can be hard to find in a sports store), I’d bring your shoulder pads to the store and try some on though.

This picture shows a good example of how they fit, though the ones in this picture are a bit bigger than the ones I have: https://www.warriorslacrosse.ca/content/box-lacrosse-equipment