No, don't pick an aspect ratio at all, that's the point! Just crop your photos completely freeform - no shape or size imposed at all. Just crop however you like.
Then, after cropping, go to File>Automate>Fit Image, and enter 2500 in both the fields (and click the "Don't enlarge" box to be safe) and press OK.
Then sharpen for web, and save.
So what I need to understand is the purpose of this squarespace site. Is it a portfolio of all your best photos, to promote your work? Or is it a site where you provide proofs to your clients from their session, so they can place an order?
The colour moire is so slight that all you need to do is add a Solid Color layer of shirt colour, set it to "Color" blend mode, and paint over the shirt.