Since you are very new to Rails, I would suggest that you take a an hour or two to go through a three step approach to gather the knowledge you'll need to answer this yourself.
When I am starting a new, complex project, I will oftentimes go through this three step process two or three times, taking a slightly different approach to the models, relationships, queries, and routes each time. This allows me to see where the advantages and disadvantages to each approach pop-up. It is usually in this process where I end up answering the types of questions that you asked below.
My apologies if this seems a little elementary (and I did not explicitly answer your questions,) but this is the approach I find useful with Rails' tools. Hope this helps.