You're welcome! I don't know of a list of restrictions so to speak (other than those listed in the TOS), but there are guidelines that have to do with memory usage, CPU utilization, background processes (cron itself is a "background process"), persistent processes, etc.
You can find a lot of information about these things on the DreamHost Wiki (http://wiki.dreamhost.com).
Generally speaking, if your cron jobs play well with others and don't unduly burden the server or impact other users' ability to serve web pages, you should be ok.
Certainly, remote fetching of web pages over http is not prohibited if done in a reasonable manner (DreamHost has cURL, wget, and rsync available).
You are well advised to research this stuff a bit on the wiki, consider using "nice" to make your processes more sociable, and make every effort to be a good "shared-server citizen" to avoid problems.
DreamHost does have a procwatch program that will kill runaway or overly resource intensive processes to protect the server's stability and usefulness, but wiht proper planning and implementation you should be able to avoid ever encountering it.