I disagree with TakingthePlunge. You do not need to wait for them to be similarly sized with your typical betta girls. Perhaps this is the case with different subspecies, but not with splendens.
They can all be introduced together. Meaning remove the full grown girl, rearrange the tank and release all of them together. Cup any bullies and let them float in the tank for a day and that should fix any issues.
I have a girl that is almost double the size of all the others and yet she is very docile and is near the bottom of the hierarchy.
20g long NPT: 3 german blue rams, 1 powder blue dwarf gourami, 12 harlequin rasboras, and 2 otos
20g regular PT: 9 female bettas, 6 dwarf cories, and 2 mystery snails
10g divided PT: 1 male DT Halfmoon betta and 1 male Super Delta elephant ear
2.5g: 1 male Halfmoon betta