Any shape will do if you are using a manifold/stainless braid and batch sparging. If you are fly sparging though a tall round drink cooler works best at limiting channeling and such.
"6.6 or 8.4 gallon-coolers are available for me. "
Pretty much every size is available to you if you look for it.
You really do want 10+ gallons. You won't need a full 10 gallons for the average 5 gallon batch, but if you every want to brew anything with Imperial in the name, you'll be glad you have the extra space.