Yes, my S8+ fits on it very well. Important note: the mic/headphone connection is covered on the S8+. It is therefore not possible to connect an external mic to the S8+.

It should already fit. I have already mounted the Note. But I think for an ideal weight ratio you would be better off with a smaller phone. With a larger device, the Osmo has to work harder and certainly needs a bit more battery. The best thing is to try it out.