The inch per gallon rule is well known to be very inconsistent for most fish and it;s better to rely on common sense when stocking a tank. If everyone followed the inch per gallon rule then by that a 10 inch oscar could be kept in a ten gallon tank which we know is incorrect.
Inch per gallon applies to small fish and not big fish, especially scoaling, cichlids, discus etc! the rule of inch per gallon don't work as they have there own requirements, an oscar can live happily in a 30gallon by its self but a 55+ would be recommended only for the simple reason they eat like pigs, feed them feeders and the inch per gallon rule would step in and say it couldn't happen, as well as them being very dirty fish they require huge filtration systems and a really good maintence scedule and backup heaters! Especially if you feed them feeders! thats why the inch per gallon rule don't apply to them, Discus, chiclids <- they are actually recommended to overstock them in a tank putting the inch per gallon rule at shame lol(Meaning you overstock the tank), so its safe to say these fish the inch per gallon rule doesn't apply to them.... these fish require huge filtration and go by there aggresiveness, how they eat, and they still use inch per gallon rule as a guide for stalking them. The inch per gallon still applies to all fish with exceptions
~ 55 gal planted 5 discus, 9 cardinals, 2 pepper corys, 2 Juli cory, 3 Ottos. Thats a tank I own and not overstocked and puts the inch per gallon rule to shame!
Back on topic ~ To OP
depending on specimen, the inch per gallon rule would say 2-3 at most in that tank looking at adult size of females can range up to 2.5-3" this is looking at the splendin and not looking at the smaller specimen the imbella (1.5-2") both are part of the splendin family, both are bubble nest makers, I would say the imbella 3-4 would work fine but if you went with splendin then you might find it difficult to keep them at bay! Imbellas are more peaceful than splendins, so really it depends on specimen that you are going to get. Before thinking of doing a female tank you should be looking at the specimen you want to keep especially being a begginer, and especially attempting a community of females.
Splending family consists of
Betta Imbellis
Betta Mahachai
Betta Smaraagdina
Betta Splenden(LFS common betta)
Betta Stiktos
Betta Rubra (Plaket)
Smaraagdiana can be housed in pairs or specie only tanks, they are not aggresive as the others and might be a good recommendation for you in that 7.5gal tank.
Mahachai can be housed in that tank to, they do require brakish water though, and grow to 2" and look like a orca don't really resemble a betta, the males can be housed together cept 1 problem they are terrified of humans I don't really know how many of these in that 7.5gal but guessing 3-4
Stiktos is kinda new to the hobby, and not alot of information on this kind of betta, they where only discovered in 2005 I believe
Imbellis is a hit or miss, as us humans breed them with other specimen we are uncertain about behaviour, the pure Imbellis are peaceful and a community tank of these females would work for sure, if mixed with splenden I am not sure, not enough reports on them being housed together, weather it be male or female, they come in short and long fin
Rubra is a run off of the Imbellis but with the more aggresive behaviour, and is known more commonly as Plaket! Its a mix of 2 and not sure what ones
Splendin most commonly kept betta and wiki has information on it if you are interested in this kind, as well as members views experiences, these are more aggresive
First decide specimen then decide if it will work in your tank would be the easier route, not all specimens are the same, especially splen and imbellis or a mixed breed of them!