FYI Terramycin is for eye infection , it is not a wormer so i do not know how you wormed your guineas with it.
Most of us use safeguard for goats 3cc per gallon of water for 3 days,make a fresh batch every day, i put it in scrambled eggs but i have no way to get it in my birds any other way as they free range plus i know they get a full dose of it that way also.
You can get fish zole at most pet stores you will need to look for something like this .
If you do not see a box like this look for the ingredient on some of the other boxes that is how i found it.
Terramycin 200, active ingredient is Oxytetracycline; can be fed to chickens, turkeys, swine, cattle and sheep. Proven effective against a variety of infectious diseases cause by bacteria. If I could post a link I would, but I am not good at that!