Do I have to use a stainless steel stock pot or can I use an aluminum one?
Stainless steel brew pots are preferable simply because of their durability. There is nothing wrong with using aluminum or ceramic coated pots for brewing.
Both of these types of pots work just fine if you treat them well.
Keep in mind that Aluminum is soft and can be easily scratched. It also is not very resistant to caustic solutions such as bleach.
Ceramic coated pots scratch less easily and tend to be resistant to caustic solutions. However, the ceramic coating can chip. If this happens the pot should no longer be used for brewing. Wort contacting the metal underneath can impart an unpleasant metallic flavor to your beer.