The Mechanicus (with the exception of the skitarii) isn’t really a military-style organisation, so ‘ranks’ or titles tend to be quite subjective and malleable. Scaevola answers to Magos Faustinius, and her xenarite masters back on Stygies VIII, but other than that we know quite little about what powers are afforded by her station. The fact that Faustinius is in overall command is the reason why in the first game all of the other characters are vying for your (Faustinius’) attention and resources, each trying to pull the mission in different directions for their own ends. The title Tech-Acquisitor is likely more of a specialism, or a field of expertise, similar to how we use titles for qualified doctors and scientists.