IMHO adding more than a single female in a 10g is pushing it, especially since you already have flagfish (which are known to be temperamental) and guppies already...and with females you cannot have less than three if you do a group. I personally think a 20 long is the smallest size for a sorority tank, but that is JMO. Definitely have a backup.
Agreed with the no less than 3- preferably 4 or more. I have a sorority tank up and running. It's a 10 gallon with 5 girls in there (20 would be fine, too), but there is a really specific way you have to do it... #1 is it really should be heavily planted/decorated so they aren't in each others' faces all the time. You need to have ALL other tank inhabitants established in the tank, or they may swarm, harass, and kill new additions after they are established in there. Then you have to purchase and add them at exactly the same time, so no one can claim territory over the tank. They then sort out a pecking order, like cichlids will do- after they sort out who is who, and all of that, they are pretty much completely peaceful together (unlike a lot of cichlids). I recommend having a breeder net for them, in case you need to give someone a time out, and stresscoat for the torn fins you will definitely see. You can still get the occasional little tiff here and there and a few torn fins, but, they usually settle down pretty quickly. I would say you'd be overstocked with a betta colony going on in there, too- and I would take j-pappy's advice about the flag fish and bettas (I've never had flag fish, but my girls are CRAZY, and can be really aggressive! Much more so than my male betta, who has his own tank.) That's just my opinion, and what I've seen over the past 6-7 mos of running this sorority tank. I agree, if you attempt it, have a back up plan.