Surface area plays a big part in it. Oak chips generally get you the most oak because there are a lot of nooks and crannies which equal a lot of surface area. Small barrels have more surface area contact compared to large barrels. They key is to take small samples relatively often (maybe every other week at first and then every week when it starts to get close) and stop when you hit the profile you're going for.
Make sure you find out when they were emptied and what kind of spirit (if any) was aged in it. They will dry out after a bit and leak. My barrel was rye whiskey, but was emptied about 6-8 months before I got it. I soaked it in water to get the outside holding a seal well, and then rehydrated the inside with 100 proof bourbon (you need at least 100 proof for killing most things). Kept rotating it every 45 minutes when I got home from work for a week.
Depends on how much coffee you want and what kind of coffee. I used about 12 oz for this, but I'm going to sample often and keg it as soon as I think it's right. Cacao nibs, I usually use about 2-4 oz per 5 gallons depending on how big a beer. Soak them in either 100 proof bourbon or rum (I prefer rum for cacao nibs) to sanitize (just enough to cover them), and pitch the whole mess in.
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