Room for one more?

I think that would probably be fine, especially since you have mostly girls. Since your cage is a little bigger than a 2x8 in terms of square footage, that seems like plenty. According to the guidelines on this site, a 2x5 is advised for three piggies, and a 2x6 is advised for four... So, following that logic, a 2x8 would be perfect for six piggies. I have ten girls and one neutered boy in a huge cage, but even with all that space they tend to stick together most of the time.

Wonderful, now I just have to make room for a quarantine cage

It would be fine, but if you could expand to a 3x7, it would even be better!

I'd love to have made a 3x7 all the way along but the cage is right next to the back door and the entrance into the kitchen. There would be no room to manouevre if I did this this. Oh how I long for a bigger house ptimist:

I think the cage will be plenty big enough for six piggies, but you could always add a second level if you wanted to... (Careful though, you could get carried away with the cage-building like some other people I know... not me, of course...)