So, here's what we figure, in a simple outline for a summary. Explanations given in paranthesis. There's not anything really major, but some of the references are a little spoilery. Oh, and more based on the events/timeline of the anime, though some generic canon facts may be taken from the manga.
-Joined the army by becoming a State Alchemist at 18 years old. (See following; also Roy's reactions at Ishbal imply that he did *not* receive much if any training as a soldier, therefore he must have started as a SA.)
-Sent to Ishbal as a State Alchemist at ~19 years old. (CANON. Roy is 15 years older than Ed according to the age given in the manga, Winry's parents were killed when she was 5, this was just a matter of math. This is also what we based much of the rest of our speculation on.)
-Knew Maes Hughes and possibly Riza Hawkeye prior to joining the military. (Hughes, when talking to Roy in one flashback scene, mentions getting a desk job in the military, implying they probably knew each other prior to joining. Explanation for Hawkeye below.)
-Attended a military academy as secondary school, entering around the end of the war with Ishbal. (She's clearly a few years younger than Roy, and it seems very unlikely that she would have started as an ordinary soldier and worked all the way up to officer.)
-Had only been under Roy's command a short time when the Elrics came to Central. (She didn't seem to understand Roy's manipulating events and other such things that were typical of him and his group at the time, while later she doesn't even blink.)
-Probably knew Roy prior to choosing a military career. (This theory is based on the dialogue from the scene where she talks with Winry, Hawkeye makes it clear she does not like shooting (and by implication, military ideals) but does so because she has someone she desires to protect. Considering that she would have been of age to enlist during the end of the war, there's no reason to believe she'd be so naive not to realize what position she was putting herself in, therefore speculating she knew Roy *before* she chose to go the military route.)
-Attended a military academy. (Hughes is definitely a few years older than Roy, but there's no way he could have been promoted from an ordinary soldier all the way to Major by the time the Elrics come to Central. So he probably went this route to become an officer, while Roy took advantage of the shortcut of becoming a State Alchemist.)
-Advanced in ranks faster than Roy. (Assuming that Hughes started as a Second Lieutenant, he must have been promoted no less than three times to reach the rank of Major by the time we see him in the anime. We're presuming he pulled this off by getting himself into a job in the military that basically handed that to him, similar to how State Alchemists are automatically given the rank of Major.)
-Knew Roy before enlisting. (Obviously Roy and Hughes knew each other from way back, and the way Hughes speaks of getting a desk job implies they knew each other before they both joined up.)
And that's about it, really. Seems short now that I write it up, but it really covers a lot of ground. Of course, there's still Havoc and all the others, but we never really get any sufficient background on them to work with. Personally I think this makes a *lot* of sense, so I figured hey, why not share?