Well prepare to be disappointed then, as Preston Garvey is very early in the game. Think of it as how other games will occasionally give a person endgame gear or powers early on as a tease for what is to come. As for your second question, who is to say Preston Garvey knows how to use power armor?
He is a military veteran and the likelihood he is proficient in power armor is very high. As others have suggested, it will likely be an early tease and then we'll need to build the suit back up as the game progresses.
Fallout 4 skimy power armor mods will be here sooner than you think.
Paladin Gunny? Yeah, he was a punk. Honestly, I did not like the way power armor was learned in the base Fallout 3 game at all. Having to go through most of the story and then go to a black out screen was pretty underwhelming. I would have preferred some obstacle course or level of training involved instead of "you are the lone wanderer and the greatest person ever, lets just give you everything."