1stTimer,
I have two barrels (5 and 10 gallon rum barrels) and brew for four others (30-50+ gallons, each). What we have found is the barrel seems to strip the body of the beer; not necessarily from the tannins either. We have put everything from RIS, Old Ale, Barley Wine, Wheat Wine, Rye Porter into these barrels....they all seem a touch thin and watery...not thick and chewy like Goose Island's Bourbon County Stout or King Henry.
I suggest that you don't let the FG get too low or to increase the body with some additional proteins, like oats, golden naked oats or wheat....without overdoing these to where they are noticeable.
I think a Robust Porter or Baltic Porter would be a great first beer into the barrel...assuming it's a fresh barrel. Let the roast (add >50% more than normal) offset the implied sweetness of the vanilla and bourbon.
That's what I did for the last 10 gallon rum barrel I received fresh; first beer in was an Imperial Porter-ish beer...heavy on chocolate malt, roast malt and gravity. I don't mind saying...it's pretty damn tasty! Just pulled Bert's Perfect Porter, brewed on the bigger side, out of the barrel recently and now have a foreign extra stout in it.
That's my experience and .02, for what it's worth.
It just occured to me now, that you might just be adding bourbon soaked oak cubes. If that's the case, ignore most of the rambling above.
I'd still bump the roast up just a tad for the reasons already stated, though.
Good luck with whatever you decide to brew!