For a newbe what would be the best pygmy octo?

Probably an o. mercatis, it's for sale at www.caribbeancreatures.com .
They list it as an o. joubini, but Dr. James Wood says it's a mercatis. :)

Just don't forget that these have pretty short livespans and are probably wild caught.

Which could mean they have even a shorter time with you. With typically a 4-6 month life span, the stronger healthier mercatoris mostly are not the ones mosty captured for sale.

I want to start a mercatoris captive breeding program due to these facts. Will keep you in mind as it progresses. However, in the interim I have decided on a baby bimac. To many possitive for these wonderful creatures to not enjoy the company of one.

How large of a tank do you plan on or currently have?

What neptune said is right, When you get them they could be full grown aldults which is bad news. Unless you can get a male and a female and hope that they will mate. :)