It doesn't mean there are only 59 apps for Honeycomb, those are just the ones that are made specifically for Honeycomb and are featured. Most apps will install on the Xoom, and the ones that don't work won't open at all, the ones that work but aren't compatible will just look weird.
The Amazon Appstore does not have a way to sort for tablet only, best bet is to do what ravizzle said, search for xoom, tablet, honeycomb, etc.
You have the option to set individual apps to automatically update, or (by default) you will see a notification in the taskbar showing there are updates available. Clicking this will open My Apps in the Market and show you which ones have updates available.
You can also browse apps in the Market on your computer at market.android.com. When you try to install one, it will tell you if it's compatible with your device.