To get this rough housing debate going...
I believe the order the Merc casters want a Galleon is as follows: Bart, PMagnus, EMagnus, Fiona/Shae, and then the rest of the human mercs..
I feel Bart's Hotshot, Batten Down the Hatches, and Feat combine to be the most optimal combo for a Galleon. Offering absurd protection and damage output. Broadsides becomes an expensive option for a second drag which may be situationally worthwhile but not a go to spell. Notably Batten Down the Hatches is an upkeep on Bart. Therefore E Eyriss shooting the Galleon is worthless for the opponent.
P Magnus brings Renegade, Snipe, Temper Metal and Iron Aggression. The feat's a loss however Magnus does have backstab and this could lead to some shenanigans.
E Magnus brings Renegade, Kill Box, Backstab (and bond), and Mobility. Calamity is also notable but awkward to use. Still it could allow a Galleon to tear apart another Collosalal in one turn.
For both Fiona and Shae it's less what they do for the Galleon and more what it does for them. They'll both enjoy having a 120 mm base to hide behind and an effecient heavy beater. Fiona can notably hide a unit of Long Gunners with Nonokrion Brand behind a Galleon. Shae can Phantasm the Galleon/Veil of Mists next to it to leading to a large area to hide behind.
Ashlynn I feel does not want to spend that many points on a single model. Quicken is also only going to be a slight speed boost if applied to a Galleon as the Def boost brings the base 9 to an 11 which isn't really great. Roulette while useful is again not as good as other options. Overall I'd rather have those 18 points as x2 heavies with some infantry.
Damiano I feel is much better off having two heavies for less then the cost of a Galleon. He'll be supurb against them due to his kit and has some synergy with the Galleon. But nothing really amazing and thus competitively not worth it.
Drake MacBain I feel the same about. Against a Collosal Drake can deliver a Nomad and rely on jackhammer to do a ton of damage. The Collosal then kills the Nomad and gets hit by a second Nomad. Then there's Kayazy Assassins.
In contrast Drake doesn't do a whole lot for a Collosal itself in comparison to dual heavies. Sure there's jackhammer but Collosals already hit really hard and can you rely on getting Galleon in melee with two heavy targets to make jackhammer worth it? Without Energizer? Upkeepable Arm boosts will be unreliable on Collosals and Drakes costs 3 focus to apply! Fortune may be the best spell he has for the Collosal but it has plenty of solid targets to go on so I don't see much of a loss there. Counter Measure's becomes awkward to use when you want to shove it down an enemies throat and have a Collosal that relies on bombarding an area with AoEs.
Furthermore Drake really wants a decent infantry combo to pair with his feat! A cheaper but just as good option in the battlegroup frees up more space for infantry.
So while I feel Drake can use it and quite well. I don't feel he particularly wants to use it and has stronger options in a combination of heavies and infantry.
So that's my perspective on matters right now. With a focus on Drake.