Of course you can mix it. It's perfectly safe to apply topically by the way. I did my research ages ago when it came to my little brother, who I love very much and would not want to steer wrong. I promise. You shouldn't worry. When I was fed up with my buildup/dry scalp situation with As I Am Co-wash, I did one last brown sugar scrub to get rid of excess buildup and dry skin.
Then like I said in my other post I took 6-8 q-tips. Dipped either side of them in tea tree oil. (I get mine in these little bottles from Trader Joes. It's pure TTO.) Then I have a dixie cup filled shallowly with water. I redip the q-tip ends in there really fast to dilute it. Then I bring the saturated q-tip ends and apply directly to the scalp, going in in between the hair at my crown, and I give myself a scalp massage using the ends. Any area that feels particularly itchy seems to calm down immediately upon application of the oil. Soft, soothing, circular motions. Try to get the entire scalp although there will be areas (particularly around the hair part, I've found) that are dryer than others. I know it sounds strange but I don't have any more flakes, dry scalp or itching. Nada. And I was really suffering last month. I couldn't sleep from the itching and burning. I thought my scalp was "healing" and that was the cause of the dry scalp but nope, it was just buildup from my AIA.
The thing about conditioner is it is a penetrating product. It's designed to be retained and to sit there. So if you're co-washing exclusively or mostly you have to be mindful of the ingredients. Certain emollients, oils, alcohols, or preservatives will sit there and irritate your skin. You have to massage and rinse vigorously, yes, but you also have to choose wisely. So find a co-wash that will not irritate you and use that on your scalp. If another conditioner is wonderful on your hair and not on the scalp then by all means, apply that conditioner to your hair
and your hair only.
2/3 of the way down, skipping the crown. Don't let it touch the scalp. Scalp and hair are different things. Co-washing can be great for the hair and bad for the scalp. So it's all about finding a balance. You should try adding a few drops of TTO to your shampoo or co-wash of choice and see if that makes a difference. If not, try the TTO topical treatment before you shower then do a refreshing water rinse right afterwards and before you co-wash (again the hair, not the scalp). It feels so good when that warm water hits your scalp and opens up the pores so the tea tree oil you've just applied can sort of tap in. For me it's highly refreshing. That's why I still do it every week even though my scalp calmed down right away. I believe it helps keep things in balance.