In a 6 foot tank you might be able to do the blue. I'd say it's right on the line though. Someone more knowledgeable will have to add their opinion. Personally, I'm not sure I'd do it although I tend to be overly cautious/anal. I've got a 6 inch convict in my 4 footer and have been debating giving him to a larger home. He doesn't act stressed but I'm still wondering if he'd be better off.
Toby's not even remotely aggressive. He's the most passive fish I have. That chill attitude doesn't hinder him in getting food though. The boy likes to eat. The flame is fairly passive with only a little aggression on rare occasions towards Toby (they are roughly the same size which is probably the reason. He never picks on the much larger convict or foxface, Sigmund). The flame picked up eating quickly.
In my experience, I have QT'd all of my fish prior to introducing them to my tank. This serves 2 purposes. 1, it allows you to train them to eat the food(s) you plan to feed and, more importantly, 2, gives you the chance to spot disease before they infect the rest of your system. A QT period should last at least 4 weeks, IMO.