You cannot create new unit types, though such a feature has been promised for SE5. (Whether it actually appears is another question...
) However, you can restrict components to specific vehicle sizes by the use of Scale Mounts. What you do is make the component prohibitively huge and expensive - say, 100 times the size and cost you want it to be. Then you make a special mount (in CompEnhancement.txt) which can only be used by your Infantry hull (using the minimum and maximum size restriction fields - make sure no other troop has the same tonnage!) and only applies to the specified Stormtrooper Armor component family (by means of the component family restriction field).