Yeah we don't support arbitrary Flake8 plugins. We do plan to support plugins eventually but the exact architecture and API are TBD.
If there are plugins that would be particularly impactful for you, I'd be happy to take a look at adding them. (We support some of the plugins you use in SQLAlchemy, like flake8-docstrings and flake8-builtins, but not flake8-rst-docstrings which I'm guessing is a blocker.)