I've seen a ton of various things about modeling, XSI, sketchup, blender, etc. Can anyone clarify my muddled brain on exactly what programs I need to, say, edit a clone trooper model that are free?

Thanks.

Get XSI mod tool that would be the easiest for what you want to do.

So can XSI open and edit .msh? I've seen a ton of topics on converting stuff. If XSI can't edit .msh, what else do I need?

you have to convert msh to obj first then you can import the obj into xsi and edit it
repsharpshooters meshtool can convert to obj