Here is how you do such a mount. Make the component oversized by 100 (Tonnage Space Taken := 300, in your case) and add this mount to /data/compenhancement.txt
Code:
Long Name := Battle Armor Mount or whatever
Short Name := Battle Armor Mount or whatever
Description := whatever
Code := BA or whatever
Comp Family Requirement := 3519 (the family number of your armor)
Cost Percent := 100
Tonnage Percent := 1
Tonnage Structure Percent := 100
Damage Percent := 100
Supply Percent := 100
Range Modifier := 0
Weapon To Hit Modifier := 0
Vehicle Size Minimum := vehicle size of your troopers
Vehicle Size Maximum := vehicle size of your troopers
Weapon Type Requirement := None
Number of Tech Req := 0
Vehicle Type := Troop