Welcome.
Regarding your question about the etiquette for posting derivatives, this is answered in the website FAQ page:
http://shipbucket.com/faq#drawings
Can I use someone else's drawing as a base for my own?
Yes - this is a hallmark of the Shipbucket format. All work posted on the site can be freely modified and remixed as necessary, as long as credits are maintained. If you take a pre-existing ship and modify it, then you must credit the original author before your name in the credit line.