So, I’m looking at this thing as a little jolly out next May.

http://www.gofar.org.uk/scottish4000s.html

Anyone able to advise on connecting (as best as possible) Aanoch Beag to the Gorms (Feshie)?

Dropping down to the ski station after the Aanochs seems obvious, but how feasible is it to use MTB’s instead of road bikes to get to the Cairngorms as off road as possible?

Very feasible, with a bit of planning, perhaps some bog trotting and a bit of climbing. There are several ways across that wide tract of land between Lochaber and the Gorm. I presume you’re planning on running the summits and biking between them? I’d respectfully suggest that taking your bike to those summits in that sequence is pretty pointless, there are many, far better ways of biking some of the higher summits than this particular combination.
If combining running and biking, this could make an excellent point to point adventure.
The route east can be chosen to be either quite mellow off road or beefed up to include a traverse of (say) the trails in Ben Alder’s high corries on the way to Dalwhinnie, as suggested on the Gofar site.

Have a look at the East Highland Way route – or the Highland Trail 550 to Laggan. You’ve then got a wee section of road from Laggan and past Catlodge to the A9, onto an old bit of Wade Road, a short section of road past Ruthven Barracks to Insh and then off-road all the way to Glen Einich.

Bike and Run is the name of the game.

I’m trying to tempt a friend away from the speed of the road bike option, so I’ll be putting forward the quickest off road/back road option I can.

So mellow tracks and wotnot seem most likely.

I’m being told the bike would be ski station to ski station.