• Subscriber - Can only edit their profile in Symbiostock back-end.
• Contributor - Can do Subscriber stuff plus create new "Post," "Image," "Comment," and "Datasheet" content and edit their own content, but cannot publish them--"Submit for Review" is there instead of "Publish"--nor can they upload new image files of any sort.  They can reprocess their own Image content from the Symbiostock Images section of the backend and insert files in the Media Library into the Description section of an Image.
• Author - Can do Contributor stuff plus add new Media to the Media Library (not proper Symbiostock Image content) and publish/edit their own content.  This means they can put a new picture into the description for content of type "Image," but not upload something that will be delivered automatically and "processed."  Not sure how Paypal works with Image content created this way ... but proceeds probably go to the person that setup the Paypal account.
• Editor - Can do Author stuff plus publish/edit stuff submitted by others.  No access to Symbiostock upload features.
• Administrator - All access.  Can use Symbiostock Upload.

Good question! You know quite a bit about wordpress. The easy answers first:

1. Yes, one contributor per site. The initial purpose of symbiostock was to get everyone independent that way.
2. Yes, one symbiocard per site. The symbiocards are generated as having a unique key according your domain, and that way they are always fetched from the root of your site predictably.

But regarding permissions - everything "significant" to symbiostock is generally done as an admin. If you find any abuse-able stuff on the lower end let me know

Images cannot be properly generated through WP's "add new image" feature (comes with registering a post type) but I have not yet figured out how to completely remove it. Technically the only legit way to add an image is through the image processor. But again if you find any loopholes or areas of possible abuse let me know.

But multiple image authors per site...with a few adjustments it should be possible. Right now everything is set up for complete independence. Once you start managing multiple authors a whole new feature set is demanded. So I started simple

the other thing to remember is that it's REALLY easy to set up multiple SYM sites -- a few people have already done ths live, and more of us have done it for testing purposes.  wp itself doesn't tequire a lot of space, and it keeps things really simple