Homebrew – Understand Why /usr/local Ownership is Unnecessary on MacOS

This can work if you also then make /usr/local/ and everything under it group-writable with chmod. The current permissions have them writable only by localadmin.

Yes, you will need to do something about the umask for users running brew. Otherwise users won't be able to modify stuff brewed by other users, even if they're both in the brewers group. You probably don't want to change their default umask, because then all the files they create would be group-writable, which is probably not what you want. You might use a custom wrapper script around the brew command which sets the umask to 002 for just the duration of that command, e.g. by doing the umask and brew calls in a subshell.

It would probably work ok to mix the two, but to be safe I'd just put homebrew in a different directory. You might run into conflicting files, perhaps in some supporting library.

I don't know what's special about your mysql build, but i'm sure homebrew has options to build different flavors. do "brew options mysql" to see what is available, perhaps it has what you need.