Assuming the fish get along well enough, I think you can get by with a 125. They're easier to find and more affordable than a 180. A 180 will give you more height, but I don't see how it would reduce aggression.
I haven't seen anyone say this but 2 SD is too few. You should have at least 6 and with getting a large tank you should maybe even get more, given the stock you mentioned.
UTEP mentioned adding more SD's a few posts back. I agree with you that a school of 6 is a minimum and with the larger tank maybe a few more would be better.