you should get one more for Charlotte! I had a female named Ella and she was very timid and shy-always hiding. Then I got her a male (Carlos) and she totally bloomed. I don't have Ella and Carlos anymore, but they did produce many babys! Belive me- two BN are twice the fun, you can watch them chase each other, play in currents, and eat- so cute!
i'm a bn breeder, so i have 7 breeders, thousends of fry, and usually around 200 juviniles. in that size tank you could easily fit a couple juviniles, but they would have to be re-homed when they got bigger then 2 inches. maybe a small school of tetras, and 2 juviniles... but if you don't have any tanks that you could rehome them, you probably couldn't fit anything else.