Is it plausible that Garibaldi, with his drinking problem, could have been security chief for as long as he was?

Well, I see no problem with an ex alcaholic as a Security Chief, but I do think that if he couldn't find that Sheridan was once married to Lochley, he's not much of a security man!

I love Bruce though, he's one of my favorite B5 characters

The chief was a recovering alcaholic while he was the Security Chief

It's a good job that there wasn't a 6th season. He kept losing hair season by season, what would have happened if there had been a 6th season, shaving layers of skin off?

I've got a question about that flashback where he's leading the defence of the station. He's wearing a helmet and says something like "This is what I was born for!" What episode did it refer to and was it actually a vision of the future?

I believe it refered to an alternate timeline in which the station was destroyed by the shadows. This timeline was averted by the events in "War Without End" part I and II.

Going slight off topic, is Jerry Doyle going for the Governership of California i.e is he a potential candidate for the post ?
Reason: I seem to recall that Jerry tried to become a Congressman, but failed.
In The U.K, the only candidates mentioned in the news (to date) were Arnold Schwartzenegger & Gary Coleman, (Mainly due to the fact that they were/are actors),& I'd like to know if he "put His hat in the Ring", so to speak.

Not that I've heard. But I understand that there's over a hundred candidates on the ballot, so he might be in there somewhere.

135, to be precise. Plus the candidate who's not on the ballot as a candidate for Gov -- Davis himself.