Would I run into any problems if I made a vehicle in XSI mod tool and then exported it using crosswalk and mshex? What exactly are my limitations. Could I export turrets too? Can I do anything to a player model at all with the programs mentioned previously? Sorry for all the questions. I would just like to know more in detail what I can and can't do with free programs and why. I could not find anything that gave me exactly what I was looking for so try to be very specific please.

Well, mshex is mostly limited to props, dunno bout vehicles, but since they animate in a way, I believe they can't be exported thru mshex. Best u got is to PM someone with the full XSI and have him export the vehicle for you.

To do that, try the All-Purpose requests forum.

Well I don't have any vehicles yet but I was just wondering. Flyers and hover vehicles don't animate do they? Thanks for your help.

vehicles don't need to have animations, so I believe you can make them in the free version

Ok, thank you.
Could I model weapons with free programs?

yes, you can make everything that not enveloped (only animated things like units, turrets and vehicles are enveloped)