sure, you can use what ever cloud server you want. But be careful. after first boot of eOS DON’T add this different NC server first run setup. Skip this step, you can add it later.
If you own a e.email account you should first setup your email account in mail app. Than you can setup your own cloud server under ‘settings’ / ‘accounts’ / ‘/e/’ with your own credentials.
If you don’t own a e.email account, you better should setup your own cloud server via WebDAv account (also under settings’ / ‘accounts’ ) and nextcloud client or what ever your cloud is using.