Welcome to the forum!
The reduction will work to attach a 36 mm hose to the rotex. (But the 27 mm will be easier in use).
My "hose strategy" is that I use the 27 mm on the sanders and the domino, and the ts55 (before my of 1010ebq got stolen on that too ; the 36 mm hose get's used for vaccuuming the floor, with the kapex, the OF 1400ebq, the planer and the ts 55 (I don't really notice improved dust collection on the ts 55, but the type of hose I use depends on which tool I am alternating with at the time.) The OF 1400 does really benefit with the larger hose, and on the planer the smaller hose won't even fit.
I have a bunch of 27mm hoses, (they came standard with the festool dust extractors, and I bought an extra one to use with a hitachi vac) I also have both the non-AS and the AS version of the 36mm, and I much prefer the non AS version, which is a lot more flexible. I also have the oldstyle plugit hose, which I got at a reduced price as a deal with the ctl 26, but I really didn't like it, and I only use it on the boom-arm (which I rarely use).