...I'm still trying to figure out why they charged me freakin' sales tax on an online store, I've not heard of a Louisiana Canon B&M store here...
Canon has a nexus in almost every state. Remember they sell copiers and office equipment.
So, question one:
Does this flash come with some type cable to hook to the hotshoe to the flash, to allow you to shoot with camera in one hand, flash in the other?
No, but Canon will be happy to sell you one.
Looking a the Canon ST-E3-RT. I found it on amazon.com for $299 shipped, that's not a bad price is it? If so, that starts me off well for this and future growth with flash off camera.
Yes that is a good price. Check CanonPriceWatch.Com for historical prices.
Questions three and more:
...would I be able to get some cheaper flashes right now to use with this flash? For instance, I'm thinking of trying to learn manual shooting (based on a great CreativeLive class the other week), could I get something like the Yongnuo YN-560 Speedlights, and have them fire off (optically?) when they sense the main 600EX-RT go off?
Only if they are compatible with Canon's optical trigger. They won't work with the ST-E3 RT, so you would need to use your 600 RT as a master, on camera or with an ETTL cord.
As with most things it boils down to personal preference. Me, I'd go ahead and get the ST-E3 RT so you can start playing with off camera flash. One umbrella and the 600 RT off camera is a great start. Pick up a big piece of white foamcore as a reflector to provide some fill light and have some fun.
Have any Best Buy gift cards sitting around? Go buy another 600 RT at Best Buy for $499 and get 18 mos. zero interest. It's still at an all-time low. Keep watching the refurb store for the next time they come back in stock for your third, fourth, etc.
Honestly, I wouldn't buy the Yongnuos. I've got nothing against them. I had their 622C transceivers and they were great. But, I think you'll just be throwing money away in the long term. If you want to go optical for awhile , you can get 580 EX IIs on eBay now for around $320-$360 used. And, they seem to be holding their value, so you shouldn't lose too much selling them later. But again, I'd just bite the bullet and put the money into the ST-E3 RT and 600 RTs.