I don't think you can send out squads to attack goblin fortresses in fortress mode. Unless you mean send an adventurer, recruit dwarves from your own fortress, and attack the goblin fortress with your recruited army in adventurer mode, in the next version.
actually i think you will have some kind of contributing military role in fortress modenext patch.
on the current dev list:
"Ability to send out fortress dwarves to lead larger groups of surrounding dwarves out around mid-level maps (or just go alone)"
if we trust today when he says he'll be finished the list in a few weeks hehe
The list on the current development page isn't the list Toady was referring to, his current list is just a teensy part of the dev page goals. Hard to say when he'll get around to army stuff, but not for another few years anyway at the very least :>
It's hard to see how the heck you'd be able to send out sufficient military forces, when your own fortress can only hold a max of about 250 without lagging out.
Seriously, the goblins send, like, fourty, every year. An average fortress is barely able to get fourty migrants every year.
If I recall correctly, the idea was that your legendary military dwarves would act like high ranking officers, and go train / conscript dwarves from surrounding hill sites. The mindless conscripts would be the majority of your army, your legendary warriors the sharp tip.
Also, the way battles work in world gen is essentially an organized line that leads to a 1v1 dual. If your side loses the dual, the next in line takes their place. This is how clowns can have kill counts in the high thousands, and how a 70 dwarf force can take on a 10,000 goblin force. So technically, right now you wouldn't need the numbers, just the skill.