1) I'd add them first, 6 is the minimum for a shoal for most species of Cories. If you add them later, they might get picked on simply because the girls would have established their own individual territory.
2) Yeah I would add more girls, for sure. 5 is really not enough for a 26 gallon, try to find five more other girls you can increase the number too if you like. Be prepared to re-home some of the girls because they might be too aggressive or that they're too weak and get picked on.
I've seen your tank and it's a NPT, the only problem I have with it is that it doesn't have the cover for a sorority, you have plants, but you don't have plants that are tall and big that break the line of sight. I suggest covering your tank with more tall plants, live or fake, doesn't really matter.
Something like this (taken from the first page of this thread):