I don't think that it is allowed for someone to port resources from one game to another.
It was said that so long as the assets being used are in both games, it is perfectly fine. And the T-51 is one such asset. Though that means that the armor is already in Fallout, so why bother converting from NV?
The rule was more made for people who want to convert their Fallout 3 mods to NV, so that they know what they can not use and would have to find a replacement, since NV doesnt have all of F3's assets.