Some people make big bucks answering that type of question. :)
One of the first areas I'd be looking at is how you've set up the taxonomy and drill down of the categories. I'd be looking at how links from the "existing pages" are being used to "funnel the juice" as they say. For example, if you take the home page and its had 20 primary cat links for most of its history, I'd be very careful not to upset the balance here. And, I'd carry that same caution to those 20 primary category index pages. Those are typically your "top level" pages. That's where the PageRankô typically starts. If you add 30 more links to your original 20, that is a total of 50 links that PR now needs to be distributed to.
If the PR of the top level categories is sufficient, you may have some breathing room. But, if those top level pages don't have the PR to support the trickle down to the volume of pages being added, I believe that is where the challenges really begin. You only have so much PR, how you "spread it" will be a determining factor.
I might consider noindexing those pages that are not considered a major part of the process. Do not "nofollow" them as this will upset the click paths. Take a look at all those pages where the visitor "needs to be" and figure out a way to get those to the upper levels of the click path so they are the ones attracting the available PR, not the intermediary pages. Don't mess with anything that is out of the norm such as rel="nofollow", etc. You can use noindex (meta robots) to your liking or you can use other methods such as IP based delivery.
Just remember, in Google's instance, it really is all about PageRankô. I typically try not use that word in my writings, the hair on my neck rises. A website with a PR3 for its home page is going to have a very difficult time funneling PR horizontally, vertically and diagonally. With low PR, you have to think more about the horizontal/vertical and leave the diagonal for later. :)