Clawhammer technique

Hi there, just created an account today. Apologies if this subject has been brought up before
I've been playing clawhammer for a couple of months and am absolutely loving it. There's just one issue with technique I think I may be doing wrong though.
When I play a melody note with the back of the middle fingernail, my thumb doesn't come down to rest on the top string. It only comes down after the strum. I've seen videos of players bringing the thumb down onto the top string with every stroke but only actually playing it after the strum. Am I doing it wrong? Thanks

Also, is it ok to strum using the same single finger you use to play the melody notes (in my case the middle finger)?

many thanks...