You can add girls individually but I suggest you start off with at least three first and then you can add girls in after but I also suggest QT'ing them for at least 2 weeks and let them float in cups or something in the tanks so that they can get used to the new girls.
With a male, he can be in first but when you add the new fish, take him out, rearrange things and then you can add him back in when the new fish is set. This way he doesn't have his territory to defend and has to find a new spot. I don't suggest nippy tetras or barbs though (unless it's cherry barbs) bottom feeders like cory's are great with Betta's because they occupy different levels of the water.
And no I don't really worry because I take time to watch to make sure they're fed. The boy's get one or two pellets at a time and I go down the line of tanks and repeat it till they each get the right amount of food. For the girls I just plop in some NLS pellets. Some sink to where the cory's get them and start feasting. I also take care to use at least one bottom/algae wafer a week at least so that the cory's can get other nutritional value. Otherwise they just scavenge all day along like all fish and they'll find something to eat always.
Sometimes I use NLS flakes in the sorority and they usually can't get them all so they sink to the bottom and the cory's scavenge for them all day long so no, I don't worry