Where SIDE/All.lvl is referenced in mission.lvl under the Hoth map, All_inf_basicsnow refers to your trooper, vanguard, pilot and scout classes and All_inf_smugglersnow refers to your wookiee class. I tried changing all_inf_basicsnow to all_inf_basicurban to change the models to urban map ones, but I guess I missed a couple of things...
It looks like the specific unit textures/models for each level are defined in All.lvl, I'm going to try changing it there.
P.S. If you check the map on the character select screen you can see where I swapped an imperial ATST for the snowspeeder, and the TaunTaun for the ATST.
EDIT: It occurred to me that it'd be a fairly simple process to compile map 'patches' to change the specific bytes we're changing here. Not something I'm going to be trying just now though.
EDIT: Just below SIDE/all.lvl and SIDE/imp.lvl is AddUnitClass and a list of individual units for each side, removing any of these will remove that class from the map - possibility for a way for server admins to restrict certain classes on certain maps? Sniper Wars map anyone?