While Imperial Spacetroopers in Terminator armour is visually awesome, I don't think they'd field many, if any..I imagine it'd be possible to modify the Gallop to include some of the adjustable mobile suit racks used on later versions of the Musai to allow the use of Dom or other-sized mobile weapons. In fact, I imagine that this might become standard on Principality of Zeon Gallops.
Also check this out: Star Wars Legion Models - Page 2 - The Piazza
These are meant to be regular Spacetroopers, but in Coreline? Well they could very well be Spacetroopers in Terminator Armor.
Regarding Spacetroopers, I imagine them to be among the most prominent users of mobile suits in the GFFA. This applies even more to Spacetroopers in the Milky Way galaxy.
1) Expensive to produce and acquire the materials to build to spec.
2) designed for much larger than average troopers
3) How many holy wars would the Imperium or Adeptus Mechanicus wage to recover their holy tech? Which could be a good factor in someone's campaign.
Although once I get the terminator armour stated, there is nothing to stop me from creating some Imperial versions for fun.