1. pretty much everthing can be used except a hockey stick
2. Lacrosse gloves only go to 13" but the sizing is slightly different.13" will fit you fine. but if you do have big hands and 13's dont fit brine gloves tend to run a bit big/loose while warrior and stx tend to make tight fiting gloves.
3. The first thing i would replaxce wuld be either gloves or amrguards. hokcey ones tend to be stiff and hard/protective while lacrosse stuff is much more flexible.
the lassst thing i would replace would be your shoulders pads. while there are a bit bulkyier they are the most similiar thing to lax pads. and if you choose to wear rib pads dont buy the hard plastic kidney/back pads. they tend to be uncomfortable to run with. get flexible regular rib pads like the brine elements or warrior mpg's. for a stick i would probably recomend shaft- stx vanadium/warrior platinum both light, durable, and reasonably priced. as for a head i would get a warrior mojo or brine cyber. to buy these things check out lax.com some of the reviews that are there are completely retarted and make no sense but some will be helpful. also lashopper.com has review of lax gear.