No, you don’t have to change your nameservers to use our hosting. We do recommend it, because it’s the easiest way to setup your domain, but you’re not required to.
If you are using other nameservers, that’s fine too. You can add your domain with the CNAME verification method:
Once the domain is linked to your account, you can update your domain’s DNS records to point it to our hosting:
You are using Cloudflare, which is perfectly fine and supported, however Cloudflare makes it impossible for us to verify whether you have configured your DNS settings in Cloudflare correctly to point the domain to our hosting.