Does deleting default group of user break anything?

I just moved my websites from shared hosting to a VPS. Of course all my users have the same default group. I need to isolate one user account from all but one area of the website. The simplest approach I found was to remove the default group from this user and give him/her a new group, and change the directory and files in part of the website the user needed access to the same new group.

I thought I read somewhere one cannot remove a user’s default group. But I did it easily using usermod. So far I see no problems. A small point was if I shift the user from shell to sftp and later back to shell from the control panel, the default group comes back. Not a terrific problem.

Here’s my question. Have I broken anything by removing the default group of a VPS user and replacing it with another group?