I have a Pages project with Cloudflare without a custom domain name assigned to it and I would like to configure some Rules. Is it possible to do so? Or I am forced to configure a custom domain and create a new website (Rules seems to be only active for each website).
For Transform Rules or Page Rules though, you would need a custom domain attached and configure it through there. You can achieve a lot of the same things through Functions however (info here: https://developers.cloudflare.com/pages/platform/functions).