Should I buy this octo?

[408mike]

Local fish store has 2 dwarfs and I want to purchase one. I have a 12 gallon nano and a 20 gallon acrylic, the nano just has tons of live rock and sand, some hermit crabs couple of other things only one fish. I was going to spend the next couple weeks building a tank specifically for an octo, just don't want to pass up this chance to get one and get started on housing them etc. Couple questions.

Can I keep it in the 12 gallon for the time being? It's a baby dwarf, I plan on having the other tank ready in a couple weeks

If I get it, how do I check the water quality? What will kill the octo? Also whats the right temp for these little guys?

Is moss bad for them? The tank has some moss in it, not much but I'm new to the whole saltwater fish thing and falling in love with it, still learning.

Just lay out for me what would be my best course of action and everything, going tomorrow to check the little guy out. REALLY want to get him.

Oh one thing if I do buy him, how do I acclimate him to the tank? You can't just dump them in the tank correct?