NCCN Clarification

I am waiting for my Oncotype results and was looking at the NCCN guidelines for Hormone Receptor-Positive HER2-Negative IDC. I find it interesting that for tumors with micromets with a size (including microinvasion) that are less than 0.5 cm that the standard of care seems to be adjuvant chemo followed by endocrine therapy. However, tumors larger than 0.5 cm call for Oncotype testing. Why is the standard of care for smaller tumors with microinvasion chemo, but larger tumors with microinvasion are recommended ODX testing?

I'm also scouring sources to try to find studies that support use of Oncotype testing in node-positive pre-menopausal women. I'm nervous about hanging chemo decision on a score that doesn't have substantiated studies for my particular situation. My oncologist is adamant that a low Oncotype score rules out chemo for me, but all studies I have come across are not looking specifically at pre-menopausal women.

On that note, how difficult is it to get a second opinion. I don't even know where to begin...do I just call other oncologists in my insurance network and tell them I'm seeking a second opinion?

I appreciate all help/insight!