First, I am NOT an expert on Poultry Illness and Injury - what I don't know about those subjects would fill books, and does.
Second, a quick glance at the research suggests that Tylosin resistant strains to both e. cecorum and g. anatis are relatively wisdespread, so that's out. One of the things I was reading suggests that e. cecorum still has not developed widespread resistance to the "-cillins" (penicillin, amoxicillin, ampicillin) so I'd be reaching for the vial of Procaine G I keep around as a general purpose go to. I found less information on g. anatis resistances to the -cillins in poultry, specifically, but a test of g. anatis isolated in cattle found it was susceptible to amoxicillin - so there's hope.
Third, do you know the pH of your water? Both species are opportunistic and colonize portions of the digestive tract. There is significant research suggesting more acidic environments help in controlling bacterial load of both (in broiler chicken studies, at least). This is a case where the use of fermented feed, some live yogurt, or (live) ACV addition to water may be beneficial [I don't consider either practice to be a general purpose cure-all]. Your target pH is 6.0 to 6.5. Below 6.0, the antibacterial properties of a lactic acid producing bacteria common in fermented foods drops considerably.