is it possible to `git status` only modified files?

You can't do this with git status, but you could use git ls-files -m to show all modified files.

It looks like git status -uno will show you only files that git is tracking, without showing anything else in the directory. Not exactly what you asked for, but perhaps accomplishes the same thing (getting a readable-length list of files that git tracks).

For modified files:

git status | grep modified: