It depends on your project size, but an SSD is unlikely to make a huge difference in compilation speeds - it's almost entirely a CPU-bound task. However an SSD can still make a big difference in your productivity with Xcode, and OS X in general. It particularly helps when you're indexing files in a project, or searching the documentation.
You'll also notice a big improvement whenever you're swapping to disk (e.g. if you're doing another task while a big compile is happening).
Unless money is very tight, I strongly recommend getting an SSD. You can save a lot of money buying a 3rd party SSD and installing it yourself (which is pretty easy in a MacBook Pro) - prices have been dropping sharply lately, I'd estimate about 50% from where they were at the start of the year.
No, it cannot. Actually, there is no there is no drive-bay at all (standard or optical). The traditional, 2.5'' 9.5mm internal notebook drives (whether SSD or HDD) that you're thinking of are not compatible (internally) with this model.
This is what the solid state "drive" looks like inside late 2013 models:
As you can see from the iFixit photo above, the SSD is really just a stick of flash memory, connected via the PCIe bus. It's made up of 8 identical NAND flash modules (in densities of either 32, 64, or 128 GB). The 256 GB drive is pictured here. There are 8 32GB chips in total, 4 on each side. The chip density corresponds to the listed drive capacity, so 8x64GB Modules = 512 GB, etc.
Though it is possible to replace the SSD with a larger capacity one, there's currently no aftermarket upgrade available yet:
Unfortunately, the proprietary PCIe 2.0-based SSD in the "Late 2013" models is limited to a smaller "blade" option, but upgrade options no doubt are forthcoming, nevertheless.
Soon forthcoming, indeed.
Consider purchasing an external storage device, or exploring 'cloud' based storage options.
Best Answer
These particular Mac Pros use an Apple designed PCIe 2.0 x4 interface. If you're wanting to replace the internal drives your options will be limited.
I would check out the following instructions provided by Apple to determine your happy to proceed. If so, I suggest you purchase your SSD from a reputable retailer. For example, the following are some well-regarded sites:
By buying from a reputable store you'll be sure to get an SSD fit for purpose, regardless of manufacturer. Being reputable means they're only going to sell something that works and they'll stand behind the product they sell, regardless of the brand. I would also check out Amazon as well for particular models once you know what you're after, as you'd probably find you can buy cheaper from there.
Another option
If I was you I would seriously consider keeping your current drive and using an external Thunderbolt enclosure to increase your storage capacity. If speed isn't so important, then you could use external USB drives instead.