Hi again Tammy and the information you obtained which says that each betta should have at least 2.5 gallons of water is absolutely correct.
Having a filter which causes some current is fine. The main thing to avoid in a betta tank is excessive water movement, current and especially turbulance.
The redness you are seeing on the fish is an indication of an illness called
Hemorrhagic Septicemia which is essentially internal bleeding.
Septicemia is a bacterial disease and as you mentioned that you started the fish on
antibiotics and antifungal medications it's good but make sure the medicines are designed to treat Septicemia. The product label should indicate if so.
A female betta darkening is a sign of a bacteria illness and/or a reaction to stress probably caused by the move/transfer.
Generally it is not advisable to house female betta(s) with male betta(s) in any ratio unless you plan to breed them as the male will beat up and sometimes kill a female during the courting/mating ritual. The only time the mating ritual might not occur is if the fish are young=1 year or less.
Please remember that the best preventative against fish disease is good water quality which can be acheived by doing weekly 25% partial water changes. It also helps bettas to maintain a steady water temperature of 80f degrees.
I'm not sure if your tank is a newly set up one but if it is, you should read the link below regarding tank cycling.
Best of luck with the bettas and please let me know if you have any questions.