Ok so I know this is last minute but this just occurred to me. We have 6players plus Chris as ringer. We were intending originally to play 6rounds. This will create a slight problem. That being you'll have to play someone twice in 6 rounds(6 rounds, 5 possible opponents)
Assuming no one drops, or no one extra shows up I might suggest these two alternatives:
1) We play 7 rounds not 6, this will let chris play(everyone gets one buy), Everyone will play everyone(6 rounds, 6 opponents). This is my Preference as it'll let Chris Play and 600 pts is quick(I bet almost everyone is done more like 30-45mins). 7x 50min is roughly equal to 6x 60min rounds anyhow.
Since Everyone plays Everyone, rounds don't matter much at all. No need to worry over pairings(actually this is true no matter what). We could even play without formal rounds theoretically; You just wait until someone you haven't played is free.
2) We play only 5 rounds, Everyone plays everyone else, Chris is just TO and sits out. Pros: simple, we're done a bit faster. Con: Less games, Chris Doesn't get to play.