Well Lando seemed to have some kidn of implied experience... Kind of weird though that they never get into that, but yeah. Han, I dunno. He was supposedly of some rank in the Empire of course but I never got the impression it was high enough to justify "general". Then again Riekean loved him, so who knows. Maybe that's all it takes...
To be fair, field commissions in military conflicts aren't all that uncommon in history though. A man's experience in a previous war could often earn him immediate high rank, such as in the American Revolution, where leaders with any experience were in short supply. But in Star Wars I kind of like to think the Alliance didn't have this shortage of experience, they just had a shortage of men in comparison to what they were facing, ya know?
I always have looked at Tagge's position though, as being over a greater force, including the aforementioned "Starfleet". In WW2 for instance, Eisenhower was the Commander of the invasion forces for D-Day... Including naval command, which had an Admiral in charge of it, but who answered to Eisenhower. So a General can command all phases of a military operation, and I just assume Tagge's that kind of guy. It's not really a bad explanation then when you think about it, but it's pretty typically covering up the "flaws" that turn up.
The only time I can think of the actual rank of Commander being used in the movies, or at least it seemed that way, was Luke being called "Commander Skywalker" by the troopers on Hoth.
There isn't really much mention of any rank outside of General when you think about it (I can only think of Captain Antilles being mentioned in ANH, and Cmdr. Skywalker in ESB), but there's also very little interaction with the lower soldiers either, except pilots.... well, that and the "Deck Officer". Lots of implied rank I think, but nothing really ever said, anywhere.
Vader maybe mentions something in ANH aboard the Tantive IV to some of the guys he's chatting with but I can't recall off-hand if they're ever called by any specific rank, or possibly specific rank.