Hi again,
Pretty much any drug that a veterinarian prescribes for a reptile will be "off-label," like your clindamycin for dogs and cats. That's because it costs the drug manufacturer time and money to get those statements verified by the FDA. So no, your vet isn't crazy. Anyways... Clindamycin, like any antibiotic, is effective against certain kinds of bacteria (you might have heard the term "broad spectrum" - that means the antibiotic is effective against many kinds of bacteria). The only way to know for certain whether an antibiotic will be effective is do a culture and sensitivity, which is what it sounds like - a lab will culture any bacteria and test whether that bacteria is susceptible to a variety of antibiotics. Lacking that, a veterinarian can make a best guess based on her patient's history and presentation.
I couldn't tell if you were interested in trying the Clindamycin with Guinness, but I had a few thoughts. First, this is a liquid medication? Most of these have a very short shelf life once they're reconstituted from a powder form, and you should be able to read this information somewhere on the box or insert, if you still have it. Second, we don't know whether Guinness has a bacterial infection, and if so, whether Clindamycin would be effective against it. Third, it sounded like Guinness reacted pretty strongly against it before, so it'd cause stress. Fourth, all that aside, it may not hurt, there's no way of knowing in a forum like this.
Hope some of that helps.
Bryan