Your 600 EX should have a slave mode so that it can be triggered with another flash.
As far as soft boxes, you can get away without a strip box by covering the box with black cloth if necessary. It depends how you need to control your light.
In regard to a softbox recommendation, it will really depend on your budget. I recommend you check out BHPhotovideo to get an idea of what is out there, as well as pricing.
Depending upon how involved you become, you will learn that light control can be complex and fun at the same time.
One more thing, be sure that whatever softbox you purchase has a grid that can be purchased as well to reduce light spill.
Yes it does have a slave mode, but it will only operate in ETTL in slave mode when being triggered by the pop up flash. I prefer working in manual, but couldn't use the flash off camera in manual without a trigger (or cord etc etc). Normally pop up flash triggering a flash gun only allows Manual as far as I know, which is why the 7D/600EX combo is weird.
I've been reading books and watching workshops for a long time now, just having some fun putting it all together
Recently watched Zack Arias's One Light workshop, so so amazing.
Will post some new shots soon