Actually, as much as I love XCom, in this case I think they are making a huge point out of not very much.. The "Alien encyclopedia" is merely the UFOPaedia (or is it USOPaedia in TftD? I need to watch more of JStank LP..!).
In the UFOPaedia, there are articles that talk about some of the aliens, which are mutants, and some of the alien technology, some of which is used to make mutants, maybe, so the "Alien encyclopedia features mutation technology". A bit underwhelming, really.
If you were thinking about mutating your soldiers, I'd say it would work very much like XCom:2012, except that the engine is not really able to do much more than buff stats, maybe have a "flippers" mutation that allows the soldier to "fly" underwater regardless of which armor they are wearing, or give better underwater vision range in the dark through a sonar mutation (like dolphins).
