Trust me, if you want to make great addons, you're going to have to use new textures and stuff, and force people do download it and use it (really, in this day and age if people can't download a 2 meg file, then that's their problem).
Skin textures don't show up in MP games if you create them yourself, even if you use textures from Res2.gob or the equivalent in MOTS. I wish this were not so, but that's it.
You play as the skins in MOTS, or you make skins that others have to download, simple as that.
Of course, you can still play as any 3do in the game, but most of these don't have the proper pups, so they look messed up, or behave erratically, and a few crash the game.
As far as editing the crosshair, yes, you can do this, I assume you mean the normal crosshair, not the Scope one. The Scope one is a BM (use BMut and a paint program that handles BMPs to edit it), and I think the normal one is also a BM, but I'm not sure which one it is. I've made my own scope crosshair before.