I'm actually on the fence with differin right now. Seems like it might be too irritating to my extremely sensitive skin. The doctor didn't really seem to care about what I had to say either. I wonder if just using the phosphate gel would suffice. I don't really know if it is imperative to use the differin and the phosphate together.
I've been on all of those before and never at the same time. Monocycline worked well for me but it did take some time and didn't clear me completely but definitely improved acne. My face could never handle topical a well so I didn't ever get the full effect of them but differin worked fantastic for my cousin. And the clindamycin phosphate worked well for my brother.
If I were you I would try to use them how he prescribed them and if its too irritating for your skin try using the differin every other day or 2 or 3 times a week. That's how my dermatologist told me to use my topicals. And if they irritate your skin too much then obviously only use one. And for me I found that gels irritated my skin more then lotions, and I had the lotion version of both of the topicals so you could also ask your dermatologist about those.
Did your brother only get prescribed the phosphate? Did it work for him for a long period of time like 2 or 3 years? If that's the case then I guess I can only use the phosphate if the differin doesn't work out for me. The Phosphate already seems to be working actually