Sub Enclosure Size

I would highly recommend going by the manufacture's suggestion. Grant it there is alwasy room to fudge. You may encounter differences in the driver's performance. For a BROAD example; using a sealed box that is smaller than recommneded will result in less low end power but produce a more punchy bass that will perform better in the higher frequencies. The opposite theory will apply to a larger box. I wouldn't recommned deviating too much in volume though.

Port sizing is more important and there is no geusstimation (is that a word?) with this, you need it to match the box and woofer requirements.

I might suggest you contact the tech support for the woofer, they can often times model up enclosures for you if you give them the space limitaions.

Like, if I were to use an 8" port in a part that is only, say, 10" deep, is that a problem?

It could be if the port diameter is greater than 2".  Leave space of at least as great as the port diameter between the end of the port tube and any obstructions.  In your case, a bend in the port...or the port exiting another side of the box, could be the solution.  Look at this pic copied from WinISD for the reason why:

The free end extension is actually a part of the acoustical port, which means:  that space has to be there.  The physical port...the part that you construct and can actually see.. is just a part of the actual port.  Leave space of at least the diameter of the port so that you can be sure you have left enough room for the acoustical port to function properly. 

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

If you are off by a little, like a couple of tenths of cubic feet, don't worry, besides you can add some polyfil and get different results w/o having to rebuild it.

Keep in mind if you go too big, you can always add interior space with mdf or soemthing.

There is a simple but not so precise way to measure internal volume by using packing peanuts, check it out on the fiberglassing forum up top.

By the way, I didn't mean to discourage you from building a ported box, I like them too.