5 gallons really is a little on the small side for a group of females. I keep my girls in a 44g along with some of my catfish, cories and a female angel. So saying I currently have only 2 girls, who disliked each other previously but now get along quite well. Previously the group consisted of around 9 girls and they had a firm hierarchy in place that was rarely challenged. If a new girl was introduced she was generally sorted out by the tank boss and slotted in wherever she fit. Some of the girls had firm friendships and tended to swim and play together (yes, they really did play!). I've also found the girls are less worried by some surface current then the males (Possibly due to the much smaller finnage making swimming easier) and have watched them play in the current at times. So long as their are places where the current is little or non existent for escape. Also they love sleeping in floating plants and having hidey holes if they want some privacy, which would be difficult, if not impossible, in a 5 gallon.
They can live without a filter. Squidge, my blind girl, who lives on her own in a 3g, has no filter. But the extra waste of having more than one in that size tank is something to be careful of and would warrent more frequent water changes then you would need for a single male.
Personally I would say use the 5g for a male and get a bigger tank for the females and have a reasonable size group. That way you can watch their interaction with each other and see some very distinctive personalities within the group.
Before I lost the majority of my girls I spawned with the tank boss, Teelie, and during her absence from the tank, she was missed by many of the group. When she returned to the tank she immediately returned to her role of tank boss with no issues from any of them.
It truly is a wonderful sight to watch the interaction of a group of females and can recommend it to anyone. In a big enough tank with a big enough group even the most aggressive of girls will calm and find her place in the society.
P.