I'd really try to find a 10 gallon if you can. 7 gallons is going to max out pretty quickly if you want to make anything bigger than a pale ale(or bitter, blonde, etc.).
You also do not have to use a Rubbermaid, you can use any type of cooler. many of the people on this site use square coolers, as they are available with larger capacities. Rubbermaids are used because of the spigot - they are ready to convert. However, a different type of cooler will work just fine, but might require a bit more effort to install your braid, bullet screen, SS shank, etc.