I've been asking questions about this elsewhere, and between that and a few games this is the 12 I've settled on;
Ballista, Pin Vice, Mainspring, Colossus, Compound, Harriet, Hoist, Ratchet, Salvo, OVelocity, VVelocity, Gutter
Locus is very hard to kill but easy to ignore, and just about everything he wants to do is covered just as well or better by other models. Which is a shame, I love his sculpt.
Mother may be useful if you get bogged down in a scrum, but Mainspring is almost always a good choice for mascot, and he can do ball retrieval and damage with only one or even no influence.
Generally, my six looks something like this;
Ballista, Mainspring, Colossus, Salvo, [OVelocity|VVelocity], [Ratchet|Gutter|Compound|Harriet]
In that list, you usually have VVel and either Ratchet or Gutter. If you run Gutter, you use Ballista and Salvo to control engagements while Colossus, VVel, and Gutter gang up on a target. If running Ratchet, Ballista's the damage dealer. In either case, Colossus can pivot to damage if the target is already KDed, and both Colossus and VVel can gun for a goal if needed.
You switch to OVel if you think you'd have trouble favorably controlling engagements; she can pressure the ball reliably and has some of the best escape tech in the game.
Compound comes in against teams you don't really want to fight in a scrum with and who will want to pressure the ball. He doesn't come in against Fish though, for example; they're good enough at dodging around to be able to play around him. Bring compound against folks that have to deal with him. As yet I'm not quite sure who I'd do that with - maybe Blacksmiths? On the other hand, Gutter can just delete Apprentices, so I'm not certain.
Harriet is a tech piece mostly for If You Can Dodge A Wrench; you use her if you know you need to have the ball but your opponent will have lots of CC.
Colossus is a very good combat support piece, but consider switching him out if your opponent could try farming him for Momentum and you don't need his crowdouts or Singled Out - generally, if you are trying to avoid engaging. I'd probably go for one of the models I passed over in the last slot if I did that, or maybe Hoist.
That leaves Hoist and Pin Vice; Hoist comes on occasionally in the last slot or in Colossus's place in this setup. You do that if you really need additional copies of particular plays. Personally I find Hoist only really works for me in lists that are very Influence-hungry (and hence inefficient), or where a character has two plays I want to use but the model hits its INF cap too early; I would consider running Ballista-Mainspring-Salvo-Ratchet-Hoist-OVelocity and buddy him up with Ratchet, for example, so I can do both Tooled Up and two Blast Earths, but just about everyone on that team wants influence so I'm not sure that's the best layout.
For Pin Vice, the general wisdom I've encountered is that the combination of her playbook and her 1" Melee makes her too easy to counterattack. I have had some success with her gunning for goals, but I suspect you'd have to be careful about who you choose to use that against - 2" Melee is not fun for PV. Harriet is very good with Pin Vice, since you'll probably be doing a lot of passing and hence momentous actions, so her stock goes up a bit in that case