@sunrise.superman, Are they molting? If molting that will affect what de-wormers you can use. Can you try to do a fecal? To do a fecal, get fresh poo from the thin ones, stick it in a ziplock baggie, stir well, and take to a vet to have the test done. If positive, that will tell you what types of worms they are. Once you know what type of worm you have, you can decide what wormer to use. If you want to de-worm without a fecal, that's okay too.
According to some studies, ivermectin is not an effective poultry de-wormer. If you want a truly water-soluble de-wormer, your choices levamisole or Wormout gel, but I would not use either of these.
I would be concerned about these birds too.
If they are not molting I would use Valbazen or Safeguard orally, but if you don't want to do them orally, and if they will eat a mash, you could make a de-wormer mash instead. Let me know if you're interested in doing this and I will give you instructions.
The rest of you all make very good points, but here are my thoughts and I hope they make some sense...
A person has no way of knowing what worms they have or don't have in their flock unless they do routine fecal exams, and necropsy every bird that dies. Me, I've done fecals, all have been negative, yet I have necropsied a few and found large roundworms and cecal worms, so I know for sure that I have large roundworms and cecal worms.
So what do I do? Because peafowl and turkeys are susceptible to blackhead (histomoniasis), which they get from the cecal worm, my peafowl and turkeys get wormed more often than the chickens and ducks do. And because I have had several false negative fecals, I no longer do fecals.

Maybe once I get my lab set up I will teach myself and start doing my own.
Some of you are saying don't de-worm unless you know they have worms and what those worms are, which I *would agree* with *if* they were all in perfect weight, but 1/3 of
@sunrise.superman's flock is underweight, and to me that says something is not right. Is it worms? Who knows, but if I were in the OP's shoes and had 5 or 6 out of 15 that were thin, I would de-worm them, and I think many vets might suggest doing that too. I know my vets would.
This is what a vet wrote:
"Prophylactic Deworming
I have been saying this for years (because I have been seeing this for years!) and here it is: Many young and adult birds can be harboring ascarids (especially if they were parent-raised for any length of time) and you can run fecals all day, and guess what? The fecals will almost always be negative. A paper out of the Univ. of Georgia a few years back confirmed this. Many a time I have been a hero when I have dewormed a bird during a second or third opinion, when it passes a pile of ascarids and shocks the owner after being repeatedly told by other vets that "the fecal was negative." Well, deworm it anyway, with something safe. I routinely use pyrantel pamoate, since you can't hurt a bird with it, unless you aspirate it!"
Source:
http://www.exoticpetvet.net/avian/challenges.html
@sunrise.superman, if you can, get a food scale from Target or Walmart and get baseline weights on all 15 if possible. If you can't do all 15, do just the thin ones. While you have them caught, check them for lice and mites and give them a basic exam. Do you know how to make spreadsheets in Excel? If so, you might find it helpful to make a chart and track their weights. This is what I do with all of my peafowl and turkeys.