Just built my first EDH cube and I want to increase the size of it soon, (it's currently 540 cards). I see a lot of people including one colored legendaries into their cubes and I'm wondering how does this typically work?

Are they part of the regular draftable cards and not included in the seeded commanders? Do most people allow them to be partnered with another mono colored legendary? Partnered with a Prismatic Piper perhaps?

Just curious as to what the most accepted way of including the single colored legendaries into a cube would be. There's just so many good creatures that could potentially be added to enhance the cube with more power.