I have 6 Sevs in my 55 corner unit (along with rather a lot of other fish - like 3 sailfins - 2 oscars - 2 parrots - etc). They seem to get along with most things, although brief squabbles can break out, it soon settles down again. In a few weeks my 75 will be up and running, and the population will be split into two groups - mainly by agressive tendency.
The Sevs will go into the 75 with 2 of the plecos, and 4 blue acara which will pretty much fill the tank.
But I reckon 6 sevs in a 55 is OK - full, but stable enough - they don't make a lot of mess (compared to Oscars for instance). Just make sure you're slightly "over filtered".