I'm new too so I want to see other responses, but I think it is fine to use a sulfate-free shampoo. Maybe experiment with using it every time or every other time and see what works. My two daughters and I are experimenting and are only 1 week in. At the moment we are starting simple-be sulfate and silicone free, comb when conditioner is in, scrunch dry with tea towel. DD14 has been using sulfate free Loreal shampoo and looking great. DD11 used it the first time this morning (had co-washed earlier in the week) so I only saw it when it was really wet. I've only co-washed so far but I'll try the Loreal sometime.
I would love to get curlier. Good luck in finding what works! I actually am curlier today, but I think it's due to the Fructis spray gel, not the co-washing.