Are you sure they are going in the middle? One end of the 153s has the same cab as a 155 so shouldn't have any problems with fitting kit that end I wouldn't have thought. the 156s currently on the line have similar cabs. The tiny cab is only at the converted end.
When I was in Lairg last week the hotel owner (who was also on the local tourist group) thought they were getting some class 153s to add to their trains on the Far North line. I would be surprised if they put a 153 in the middle of a 158, it would screw the air-con in the other two carriages up more than normal, if nothing else.
Aye, it's going in the middle. Mainly so Scotrail don't need to obtain any more RETB (signalling system) sets, which are in extremely short supply! The 153s will be shunted in and out as required for summer and winter usage at the depot, so drivers won't need traction knowledge (other than fault finding/evacuation methods) on the 153s (other than a couple of drivers at the main depot in Glasgow).
(There is sufficient difference between the BR/Leyland class 155/153 big cab and the Metro-Cammell class 156 cab for the conversion course to last several days, if not longer. Driver training doesn't just cover "This makes it go and this makes it stop", there's also fault finding and location of circuit breakers and fire extinguishers, etc. Plus, if a driver doesn't drive that type of stock for more than 6 months, they have to start again from scratch! The same applies for route knowledge, incidentally!)
There is a plan for them on the Far North Line eventually (if they work out on the WHL and some more are available in "not too knackered" condition). The air-con shouldn't be a problem, as only the saloon is a/c on a 158, not the vestibule - from past experience riding in the vestibule of EMT and Northern class 158s in summer.