That's a rule that doesn't make much sense. I don't have my rules with me now - a medium BA with 1/0 would be able to swarm, correct?
Agreed, and yes, a bipedal Medium suit with just 1 ground MP can Swarm (and do Leg Attacks), assuming that it has the appropriate manipulators.
Do UMUs override the inability of larger suits to perfrom Leg Attacks?
As SCC said, Heavies and Assaults can't do any Anti-'Mech attacks, no matter what secondary motive system that they may have.
How would you calculate BA vs. BA in h2h combat?
or Conventional Infantry vs BA in h2h combat?
I know the latter would be a rare event to the extreme, but there is RL examples of desperate combat of the type.
Under Total War, Battle Armor can only conduct melee attacks on Battle Armor (and conventional infantry), if they have Vibro-Claws, in which case they do one point of damage per Claw. For example, a Point of Afreets would do 10 points.
Likewise, conventional infantry cannot inflict melee damage on Battle Armor.