Only a few MD schools require a clinical letter at all. Several DO schools expect a letter from a physician. If you are applying DO and one of these docs is a DO get a letter for sure. If you are applying DO and none of the docs are DO get a letter from the MD to use. I’ve read that DO schools will accept a MD letter.
No, you do not unless you are shadowing a DO and intend to apply to DO schools. MD schools tend to find letters from pre-med advisors/committees and faculty who have taught you or supervised your work in the lab more informative.
A general rule of thumb for rec letters is that how 'useful' they are depends on how much the person can actually speak about you/recommend you. If all you did was shadow a doctor, there's not much for them to really say, so it's not needed very much.