You can use tylan 200 with no problem. You just need to adjust the dosage given. Did you speak with someone at the store about having tylan 50? All tractor supplies will carry this at store level, it is with the cattle products. Either way, best of luck to you.
20 gauge will be perfect for tylan if you can ever find it. I cant believe TSC didnt have it. Were they just out of it? It is very easy to inject. Much easier than I thought it would be. I'm sure you will do fine.
The bubbling is not a good sign. If you have a Tractor Supply near you, you can get Tylan 50 injectable for under $10. For under $15 you can find gallimycin, erythromycin, or oxytetracycline hydrochloride. All water solubles, meaning they mix with water. Injectables will treat in 3 days...