Hello,
From what I know Rainmeter don't interfere with Start11 but I can not be 100% sure. I run Start11 and my own custom Rainmeter without issue for long time. My guess its depend on which Rainmeter skin you use. As long as its don't change the Start menu panel, taskbar and Windows explorer, it should be okay.
Thank you.
Basj,
Stardock Community Assistant.