Well, again . . . at the time there were no Clan secondline machines, so giving them Omnis to mark officers and give them a slight edge up fits. The clusters also being attached to fronline invading galaxies also provides some reasoning . . . The Falcons end up with some very solid standard Battlemechs you could assign out- Incubus, Horned Owl, Griffin IIC, Shadow Hawk IIC, Hellhound, Vapor Eagle, Rifleman IIC, Warhammer IIC and Phoenix Hawk IIC among others.
Really the breakdown of Invading Clan forces at the time should be . . .
Frontline- All Omni, all the time! This is the latest & greatest Omnis which IMO means for Falcons some of their Turkina/Night Gyr/Black Lanner/Fire Falcons should be involved while say the Jags are strutting Shadow Cats and Cauldron Born. The galaxies used to invade will not be pod-limited, though ammo did turn out to be a problem for some Clans and even varied by cluster. Per other discussions in previous threads they will also be IMO newly built Omnis- nothing over 10 years old or 10 major rebuilds, at least. Where do old Omnis go you ask?
Secondline- To the next tier! Used and abused Omnis get rotated out of frontline units when new production is delivered . . . along with damaged/rebuilt weapons or other components. New mechs can be found here for Bloodnamed and/or Ristars or taken as isorla from Trials, usually as the defender sometimes the attacker. Which Clan it is determines how many Omnis, Clan standards, refit IS machines, or Star League left overs for the clusters. Some secondline galaxies are hard to distinguish from frontline galaxies (Wolves' Tau in '57) and occasionally one makes the leap to frontline status. IIRC only one secondline galaxy actually was included among the invading Clans, which was Epsilon . . . though their description also sounds like a garrison galaxy. Most were left in the Homeworlds until after Tukayyid.
Garrison- Depending on the Clan (IE, smaller) the line of difference between Secondline & Garrison galaxies/clusters can be narrow, and its usually that secondline is barely better than the garrison formations. Garrison galaxies typically include the solahma troops. Omnis that make it this far would probably be in bad shape and be assigned only to officers or disgraced bloodnamed (Hey Conal!) with the better garrison clusters getting Clan standard machines while those on the bottom of the pile (poor Clans) get barely functional Star League or Invasion isorla (when its available).
As I said, one of the quirks of the Falcons during the invasion is that their frontline galaxies included eyrie (training) and solahma (suicide) clusters along side their veteran flagship clusters. Not sure much of that survives Marthe's reorganization during the Coventry campaign or the Jihad. It may have also been a slight-of-hand that Showers winked at for the Falcons Invasion bid. 'What? They are solahma, of course they do not count in our bid- we just said all of the Peregrine Galaxy is all.'