Even if the beer's completely fermented out, it wouldn't be a bad idea to cap the barrel with an airlock or a 3-stage bung to allow excess pressure to vent out (apparently that's what they're doing at Russian River).
As for cleaning/prepping the barrel initially, check out MoreBeer's Barrel Prep Guide, .
All in all, a month in a 2.5gal barrel should be more than enough oak, could end up harsh, though it may provide a little less depth than a larger sized barrel.
You could definitely serve straight from the barrel, as long as you've previously ensured that you're not going to be worrying about trub by either cold-racking or filtering the beer into the cask. For more on tapping cask ales, see CAMRA's guides on Cellarmanship.