Fun fact: the word 'formian' comes from 'formicidae', the official term for the ant family.
With etymology done: let's get started on entomology. Formian myrmarchs have 12 HD. They are Large and have good scores all around, with +8 to all physical stats and +6 to all mentals. In addition to default formian immunities, myrmarchs also have fast healing and improved SR.
Myrmarchs lack their lesser kin's claws, but the ability to use weapons normally makes up for that. Their poison deals 2d6 dexterity damage, making it far more interesting than the warrior's and taskmaster's (then again, poison is getting easier to prevent at those levels).
Myrmarchs also have a few SLA's, but they are almost all inferior to what a caster can do by now. The only exceptions are self-only Greater Teleport and Dictum, both of which a cleric can't use until ECL 13.
In the end, I think +0 LA is fine here.