Low on internal data storage - looks like the some apps use a lot! So, looking at the Gmail, Twitter, and Facebook apps for example, the apps themselves only take up 3-4 MB each, but then the app data is like 10+ MB for each. Is that stored data the synced copies of my email inbox, or my twitter feed, or FB feed? And, if so, is there a way for me to limit how much of my phone's internal storage each app gets allocated?
There really isn't much control you can do over limiting storage.  Most likely the data is from the apps using local SQLite databases to store information their apps use.

You can pretty much only do these things:


Move the app to the SD card if you have Android 2.2
Clear the data cache every now and then (but it's going to grow again)