Those times were pretty wild. Just bottle up random chemicals and cross fingers. Boy am I glad for the FDA.

It's a good assumption. If one thing is an antibiotic and the other is not toxic, you can assume that the mix will be an antibiotic. Perhaps the solvent will cause the antibiotic to be absorbed slower or faster. Perhaps the solvent will cause the antibiotic to be eliminated slower or faster. So you may need to adjust the dose, or the time between takes.

There are cases where the solvent will react with the antibiotic, and there are corner cases of weird interactions. Some can be predicted from general chemical rules, but some are unexpected.

So instead of mixing random stuff and following advice of morons like me in the internet, it's better to ask a real medical doctor and a pharmacologist, and make some animal and clinical trial anyway.