Am installing a PULLER electric fan, should I take off the mechanical flex fan?, or can they both run?

A good mechanical fan with a shroud can't be beat for cooling. You do have a shroud right? If you need an electric fan because of a blower or think you're gonna free up some power then don't run both.

Ditch the electric puller fan and install a shroud that covers 100% of your rad core and places the center line of the blades on the mechanical fan 1/2" inside the shroud .

While I agree a mechanical fan can and will get the job done, a well thought out electric fan set-up will do the job just as well if not better. You just have to make sure you use a shroud that covers a very large percentage of the core and select a fan with enough CFM. Much like using a mechanical fan you need to have the correct shroud. Blade engagement (1/2 of the blad in the shroud, 1/2 out) is also important.

No need for the flex fan if you use the electric puller. You do not need or want to run both. If the choice is a flex fan or an electric, I'll take the electric and it will actually cool better. Now if you have a clutch fan that's a different story and should get the job done.

Are you having a problem cooling with your current set up?

I have run only an electric fan for a while now and never looked back, just do your research and decide what you want to do