One of the safest things to worm chickens with is Ivomec given orally. It has a large safety margin. I worm my turkeys with it to prevent blackhead, and sometimes dose canaries with it for mites. It would only take a couple of drops of the cattle injectable liquid to dose such a small chicken. If you can't find that you can buy a tube of ivermectin paste horse wormer and give just what you could pick up on the end of a flat toothpick. However, that wouldn't have anything to do with the lameness problems.
Green poop in some birds can be a sign of salmonella infection. However, "green" poop in grazing omnivorous birds like chickens can be totally normal depending on what they are eating. There's brown or green stuff from the intestine and white stuff that is from the kidneys. Then every so often there's real stinky more liquid stuff that's from the cecum. If you have 2 birds affected now, and I'm assuming you paid a bit for these, it may be time to find an avian vet before you have a problem with the whole flock.
Also, you don't want to put any chicken with a problem or injury back in the coop with other birds for another reason, because the healthy birds tend to attack any that show weakness and can quickly kill them. Good luck!