Can SA buttons be assigned to pools other than the default main pool 70?

They're two different animals. SA buttons are "System Access", while pools are what you are accessing when you dial 9 and an outside number. ARS can determine which pools are available for a particular call, and you can assign a specific pool button to a phone to bypass ARS and go out over a specific pool.

OK, when I press an SA button, then dial a 9, I am now getting a trunk from a pool, correct? If yes, how do you designate which pool to get a trunk?

"OK, when I press an SA button, then dial a 9, I am now getting a trunk from a pool, correct?"

Actually, no. You're getting an "outside" dial tone generated by the Legend while it is evaluating what you dial next. Then based on what you dial, and the settings in the Auto Route Selection (ARS) tables, the system selects a trunk and dials your number for you.

It's not called "Least Cost Routing" instead of "Auto Route Selection" for a reason: if programmed incorectly, it might be MOST cost routing.

If you're familiar with the system and programming, you should examine your ARS tables and pool assignments, and refer to the manuals that you can download from the Avaya support site. If you're not familiar with them, you should probably contact a vendor to help you set them up and understand them for future changes.