My rationale for the 10 is as follows:
Hearth, Ferrite, Furnace, and Burnish for the Masters and Alloy, Cast, and oCinder for the Apprentices are too useful in too many situations to drop, locking 7 slots automatically.
Anvil has his uses, but they're too situational and he's too slow to always use them effectively, so he's out.
I personally think Sledge is great even without Anvil, but both his damage dealing and ballkeeping roles can be duplicated pretty well by other players, so he's a bit redundant in a 10.
This leaves Farris and Bolt. I love Farris for all the reasons you mentioned and I've gotten great use out of her in a large number of games to the extent where she's in my 6 at least 50% of the time. Bolt, however, only shines if he starts within 6" of Farris...otherwise he becomes much, much less useful. This really shackles Farris, whose speed and threat radius is one of her best points. Also, in my experience he's remarkably easy to shut down if your opponent has encountered him before.