if I am defending and I have a glasc mixologist and my opponent has a 2 stellacorns, and after combat resolves, can I play a galio indefatigable at the same battlefield and since there's only 1 stellacorn does that force the stellacorn to recall back to base?

Yes — you can play Galio there, and yes, the surviving Stellacorn will be recalled to base.

As the defender you control that battlefield, and Mixologist's Deathknell resolves before the combat result is determined, so Galio (cost 3) is a legal play from your trash to the same battlefield. Since both sides now have units, the result is "No Result" and a fresh Combat Showdown is staged at that battlefield. Galio doesn't deal combat damage, so the Stellacorn can't be killed and is still present after combat damage — and because a defender (Galio) remains, the Combat Cleanup recalls the attacking Stellacorn to its owner's base (Rule 466.1.a.2).