Not sure if there’s any “strict” rule about it, but the ideal configuration would be to configure the game as paid. When uploading files, you can assign some of them as “This is a free demo” and they will appear properly on the game’s page.
Marking your game as free, when it’s not, is very confusing. I wouldn’t be surprised if a lot of users were frustrated because of it, while searching for actual free games.
Also when replying to someone, make sure to click the “Reply” link at the bottom of their comment, that way they will get a notification and be able to reply back (if needed). :)