Really, it sounds to me like she is simply growing and maturing more slowly than the others. I wouldn't worm them under 6 months without more definite signs of worms, and I'm not seeing that. I certainly would not give Sulmet unless I had more definite indications that they need it, and I am not reading that. Corid won't hurt, but Sulmet is rather rough on them; necessary, sometimes, but not to be given lightly.
What I would most definitely do is give probiotics. Chicks raised by mama hens get good gut flora from the mama by eating some of their poo. Brooded chicks need us to provide this. You can buy probiotics at the feed stpre, or for humans in the drug store ("colon support" or some such), or you can simply give them about 1 tsp. of plain live culture yogurt per chick, mixed in with feed or scrambled eggs, or whatever they will eat. They don't digest milk well, so you need to limit this, but they will get the gut flora whether they digest the yogurt or not. A bit of baby applesauce in this mix is wonderful for them. Organic apple cider vinegar is, as well, added to the water. You could add a few drops of infant vitamins without iron to the yogurt/egg snack. As with people, a few supplements can make a lot of difference.
