How old is the SMZ TMP? I always have problems with doxycycline dosages for chickens, as they are all over the place if you look them up. We are not vets here, just chicken owners. Poultry DVM is not the best sight to follow, since it is a vague site run by some unknown person that sounds very official. Some of their info is wrong, although some is good. I prefer to look up articles and studies of drugs, and get dosages from veterinary drug books such as Plumbs or from an actual vet site.
I tried looking for info about using expired oxytetracycline, but most articles are 25 years old and refer to liquid oxytetracycline. There are some online stores such as Jedds.com, Allivet.com, and others where you can get fresh antibiotics for pigeons or fish. Aqua Mox for fish is a good amoxicillin to use. Baytril or enrofloxacin is one of the best antibiotics. Any antibiotic will have an egg withdrawal time afterwards.