I personally wouldn't even link to it.  Many people believe that Google still looks to see what's on the other end of a nofollow, so I wouldn't even test the waters by linking to the adult site at all.

First off, I would definitely suggest not linking the two if the non-adult site is trying to rank for something not adult related.  You are the company you keep on the internet.

With that off my chest, what kind of content is going to be on the non-adult site?

There are some minor negative correlations when it comes to length of domains.  If it's a competitive search, then it may not be worth the minor interference.  However, if it's a low competitive vertical, then go right ahead.

My opinion, though, would be to use a shorter search phrase like goingoninfrance.com.  This will give you a broader appeal to consumers by not having the "right now" part, and it is short enough to remember.  This also keeps you safe if the algorithm would change to further impact long urls.

I've talked to a couple people that have said that 301's will pass on the penalty.  A better alternative would be to try move the links over to your new website in waves.  If you experience a drop, then you know the link exists in that group.  Just do some quick checks on their domain, and see what you find.

Of course, if you are working with 10,000+ links, your batches are going to have to be pretty large.

Then again, if you don't have more than a couple hundred, go through the various domains, and try to identify problematic links.

-Maximum number of internal links on a page

Depending on how powerful your site is, I would suggest keeping it around the 100 links mark per inventory pages; at least initially.  If you offer the ability to see more products at once that will keep all of your customers happy.  However, the fewer links per page, the more power you send down to your product pages.  If you start building links to these product pages, though, that could become irrelevant - depending on competition.  If you still need to give extra boost to specific products, look for ways to link them internally from other pages.

-If Google frowns on really long category pages

From what I read, Google doesn't really consider the navigation pages to be very valuable content.  However, I would only be concerned with that if you were trying to rank your search pages.  So, I would focus on what your potential customers would want.  If you feel like you need to rank for a specific phrase, generate a landing page that leads into that specific inventory search.

-Anything else

I would suggest testing multiple designs.  It's easy for us to tell you what you should do, but it's better to hear it from your visitors.  Also, don't spend a lot of effort trying to get a specific search to rank, instead create a landing page for that product category, which will then lead into your search.  You will have a lot of unique content on there, and your customers expect to see a landing page.