A little late to the party, but as an alternative to the above, you could possibly see tighter integration in the IS units, and to get a closer scale to a Nova you could have something like this:
Nova Company:
Lance 1
Avatar
Blackjack
Battle Armor 1
Battle Armor 2
Lance 2
Firestarter
Arctic Fox
Battle Armor 3
Battle Armor 4
Lance 3
Strider
Owens
Battle Armor 5
Battle Armor 6
The usual balance post invasion is Company vs Binary (12 mechs vs 10 mechs), so I think this compares well (6 mechs and 24 suits of battle armor vs 5 mechs and 25 suits of battle armor). A little weak on the IS side maybe but if you're using C3 it gets closer.
Or, if you want just a bit more umf in the concept, you could also go with a reinforced company sized unit and do:
Lance 1:
Avatar
Blackjack
Firestarter
Arctic Fox
Lance 2:
Strider
Strider
Owens
Owens
(Or whatever for the mech choices--I'm just thinking Nova philosophy, so mostly fast mechs to move the BA around)
Platoon 1:
BAx4
Platoon 2:
BAx4
This comes out to a balance of 8 mechs vs 5 mechs and 32 BA vs 25 BA. Not only that, but 8 mechs and 2 BA platoons fit nicely to an Onion, and you can even keep your aeros and free up some extra cargo space for all those spare parts the BA's are going to be 'rescuing' from the OpFor. This also fits well in the typical IS rank structure, as you would have a Captain (or equivalent) commanding the company, a senior Lieutenant commanding the lead BA platoon and acting as company 2nd, and junior Lt's commanding the 2nd mech/BA platoons. No need to shove an extra officer into the company level pay scale, just stuff a few more enlisted men in there.
This second option also expands well into a 'Supernova Binary/Trinary' type Batallion. Dock 2-3 Onions on a Merchant or Invader and send a Batallion of these buggers as a task force. Nasty.
If you're fortunate enough to have one, you could even fit all 16 mechs and 16 BA squads of a 'Supernova Binary Company' into a Fortress, have artillery support. Hell, you've even got enough space left over in the Fortress to bring along a decent contingent of support vehicles of the repair/rearm/etc variety. Bring along a Leopard on a merchant for a wing of air support on your Merchant. Or maybe this is your scout/beachhead force and you've got 2 unions from your Invader flying in with another 2 companies of mechs...