I've brewed a bunch of Mango beers over the years, including a Mango Habanero IPA. There are several threads available to search through if you want some light reading. But to summarize what I do...add the mango puree at the tail end of fermentation in primary, about a pound per gallon works well. I puree fresh mangoes that have been frozen (and thawed) with 8 oz of vodka for sanitizing. Concentrated juice also works and you can pasteurize your puree instead of adding vodka, and some simply rely on the freezing to sanitize.
The mango will kick off some more fermentation activity but it won't increase your abv. Most fresh fruit purees have a lower OG than your beer. Lots of water in fruit. It will increase your volume, though so plan accordingly. 6 lbs of mango puree is about 3/4 of a gallon.
The best way to add habanero to beer is to make a tincture (fresh sliced habaneros in vodka), this only takes a few days or weeks to extract both flavor and heat. After your beer is finished, add small amounts to samples and find out what level of heat you want. Be precise in both the sample volume and the amounts of tincture added so you can do some simple math to figure how much to add to the batch. Then package (I keg).
Transferring a fruit heavy beer can be a challenge. A floating dip tube works great for kegging. If you rack out of your fermentor with a siphon, it's easy to avoid the fruit at the bottom, but I close transfer, so that is not an option. If you have the capability of cold crashing and dumping the fruit prior to kegging, such as with a conical fermentor, than that is the simplest way.
Mango and beer go very well together. Enjoy!
