I found some tutorials about Pydio in ISPConfig but not for Pydio Cell. Is it compatible or do I need to use traditional Pydio for WebFTP or are there any other options?
Basically you can use any webFTP client that is able to connect by FTP protocol with ISPConfig. Here are also some discussions about other alternative webFTP clients: https://www.howtoforge.com/community/threads/a-file-manager-that-works.83895/
pydio-cells is a completely re-write, it has no similarity to the previous versions of pydio the big part of using pydio, from 5.* through to 8.2.5, is the auth and storage driver used for ispconfig are based on ftp, since the ftp server storage connector or ftp auth system doesn't exist in pydio-cells, I don't see how it can work. it may be possible to write your own storage driver, or fudge something that creates a new static mount when a connection is requested and remove it after, but either of those seem like more effort than it's worth, and may not even be possible.