An epiphany of sorts has happened after the first chapter of 'Dead men walking by Steve Lyons'. This forum has made me want to read my Guard novels again.
Anyway back to the epiphany; The use of Ratlings etc would come down more to Regiment culture.
Cadians would use anything available to the Guard.
Catachans definitely use abhumans, being very useful in jungle fighting; Commissars & Psykers tend to suffer mildly fatal 'incidents'. Enginseers, few vehicles, no need.
Elysians & Tallarn, if you can not jump/ride your useless. No Ratlings here.
Krieg: "Are those mutants?" bang-bang.. Bye bye abhumans & psykers.
Commisars as liasons & little need for Enginseers as Siege not mechanised. Priest most likely here.
Vostroyans: "Are those servitors?" Abhumans unlikely. Close ties to Mechanicus mean Engineseer are the most likely over any other. More Omnissiah than god-Emperor.
Maccabians: Bang-bang Ratlings; Ogryns; Psykers; & heathen Engineseers. Commissars why need them. Priest every where.
Also, I reckon the problem with the Ratlings applies to the Krieg. How do you play a soldier who has no asperations but soldiering, no name, & no face (only rebreathers).
So love your Ratling friends, just do not 'love' your Ratling friend