I have the Boss tuner (I use it for electric bass), and it is fine. It is chromatic, and it can tune a "C", so it would work. I wouldn't recommend it, though.
For Eurorack, I would recommend a module with a built in Tuner instead. It is much easier to use in practice (I am not even sure the Boss pedal would work with modular signal levels). Two suggestions (I have both) are the Disting Mk 4 and the O'Tool+. Both are great, and both do a lot of other stuff. Personally, I use the O'Tool to tune. I can really recommend it, especially for the oscilloscope function. It is easy to dismiss a scope as being unnecessary when you are starting out (after all, it doesn't do anything to the sound), but it is fantastic for getting a window into what is actually going on in your modular.