The cream would be a pain to apply over a broad area, and it would make your hair look like a mess. I think a shampoo would give broader coverage quickly and could be engineered to not make the hair look filled with a dry cream.I'm not sure about a tretinoin shampoo. Why not just use the topical cream? Most likely highest concentration would yield highest sulfatransferase results. But to mitigate any inflammation you can start with .025%
Do you have a link to any of the studies that talk about tretinoin increasing minoxidil efficiency?