"Swell" is an older expression (about 1930s-1950s) meaning "well", "good" or "great". I suspect it's a very USA expression, so I was surprised to see it said by Harry Potter. ;) Here's a few examples: "Gee Suzie, I think you're swell." = I think you're great/a wonderful person. "Oh, that's just swell!" = that's just great (can be used sarcastically) "How are you today?" "Oh, I'm swell." = I'm good. "we will all get along swell" = we will all get along with no problems/get along great. I'm already doing the accent! >_< It's used ironically in this well-loved musical piece, "swell" roughly meaning "well-to-do": http://www.youtube.com/watch?v=Hrzo5SPaOvg

"We will get along in an excellent fashion" or "we will be good friends"

get along well