Okay, having now finished HM1RM (bar stand-alone): difficulty scaling seems alright aside from mission 3; that was the only one in the end I had to scrap everything for. By the last mission, I ended up with 9 destroyers, 8 missile destroyers, 9 heavy cruisers (all of the last two captured, I captured to my limit long before I actually got the tech to build my own). Couple of places in the protect missions, it seemed a bit touch and go, but then again I was focussing a bit on capturing, rather than destroying enemy capitals.
It is just me or are cloaks basically useless in SP? I tried with cloaked fighters a couple of times, but they never seemed to actually be hidden. (Lack of an icon indicating whether cloak is active or not doesn't help, either.)
And if I had built up to my limit of salvage corvettes, I might have actually done better with that in hindsight.