I knew TRO: Defensor Hominum was a bit different, but didn't fully understand without sitting down with TM and going through it before re-reading the Engineer's Notes at the end. Many of the units with this TRO were updated using ComStar's inventory of spare parts - which had massive numbers of certain common engine sizes, but only a couple of the 'oddball' ones found in several different units. These units were deliberately given a XL engine that was not a perfect match, to allow new weapon, armor, and equipment configurations. I'm not sure if its legal - I haven't found anything about it, yet, so I'll probably ask here on the forum.
Yes, they are! Oversized engines are possible as a FrankenMech conversion. p. 189, SO.[\i]