adding out of the ordinary flavors is a great way to make an interesting beer, but also a great way to make a terrible beer.
I see it often happen that someone wants to experiment (which is great), but they go overboard with the extra stuff. My suggestion when using something different in a beer for the first time is to be conservative with it and see how it comes out. Tweak from there.
I personally like a little molasses in my porter, but it really doesn't take much to give the beer some character. It's really easy to use too much and end up with a beer that simply tastes like molasses instead of a good beer with a hint of molasses.
For something like rum, on bottling day, just take a sample and add a little rum to it. taste and adjust until you find the right amount. Then do the math and figure out how much to dose the batch to match that level.