Honestly people, I think we're beating around the bush and really looking for an absolute fact where strongly implied facts are all we REALLY have to go on.
Yes, well, short of a character reminiscing about his or her career and explicitly saying "Back when I was Chief Engineer/Chief Tactical Officer/Grand Admiral of the Fleet...", we don't have much else to go on.
From my perspective, I've always been under the impression that Riker and Sisko both had served as Chief Engineers at one point and as XOs both on 1-2 ships before their current commands (or in Riker's case, before serving under Jean-Luc).
I can definitely see Sisko as a Chief Engineer, as his engineering background was implied. Certainly Leyton all but called him his chief engineer. Why would Leyton have picked a junior engineer to be his XO? It'd be like Picard selecting Barclay over LaForge to replace Riker.
Riker, though, I just don't see any particular engineering expertise there. His rise from ensign to commander was so meteoric that I just don't see how he'd have had time
to be Chief Engineer anywhere. He was in the same Academy class as LaForge, and one was a junior lieutenant in 2364, and the other a commander. How'd that happen? Rapid promotion through engineering proficiency?
Tactical experience might be a better explanation for Riker's rapid rise. As a tactical officer he'd be on the bridge more often than if he were an engineer, and he'd be the type to take command if the other senior officers were down. Maybe something similar to Picard's incident on Stargazer
- taking command when the CO and XO were out of commission - allowed Riker to rise so rapidly. His talents for command were probably recognized early. Having a knack for regulating plasma flow wouldn't lead to such a fast rise.