Coexistence of Homebrew and pyenv on MacOSX Yosemite

I'll try to answer each of your questions.

1 Does brew "frown" upon using pyenv's Python rather than brew's?

In short, yes. They lay out the whole story here, and in short you're on your own to manage issue that come up.

2 Will brew Python and pyenv Python conflict?

No. As Tim Smith says in another answer, "nothing bad should happen if you do". Use your PATH to manage which Python you (or an application) sees (and this is what pyenv does).

3 Packages with Python dependency satisfied by brew Python working with "oustide" Python from pyenv

As Tim said, this shouldn't be a problem.

4 Use symlinks so that we only use pyenv Python?

I think you're referring to this post, and it's a fun idea, but likely perilous for maintenance. Fwiw, I'm doing this now. I lay out how to do it here.