AFP is the default file sharing mechanism on the Mac so is the default if you turn on file sharing in System Preferences.
As to your questions:
1 & 2. You can share the Drobo between Time Machine and regular file storage. The only issue is disk space. Time Machine backups will grow to fit the space available, and over time may end up filling up your disk.
The method that has been recommended here a while back is to use a thing called TimeTamer (http://www.drobo.com/droboapps/downloads/index.php?id=16), which makes a disk image file called a sparsebundle, sort of like the disk image files that Mac applications typically come on. These files have two main features used here:
- They can be set up to have a maximum size (say 250GB)
- They do not allocate all the space at once, they grow as they are filled up with stuff up to the maximum size allocated at creation.
So what you do is create one of these files, with it’s maximum size set at the size that you want to allocate to your backups. Put the file on your Drobo, then have your Macs back up to that file. You could have one such file per Mac if you like so that one Mac won’t hog all the space.
There has been some controversy around this (see http://www.drobospace.com/forums/showthread.php?tid=371). There were quite a few people who did use it successfully though, so it might be worth a go.
Format your Drobo using HFS+ using Dashboard and you should be fine. Use a 16TB volume size unless you have a specific reason not to. There is no startup time penalty on the Mac for large volumes, so go straight to the biggest you can.
If you do hook to an Airport Extreme you won’t be able to use Dashboard, so you will have to look at the lights on the Drobo to check it’s capacity, and you won’t get any warnings for disk failures, etc. Other than that, I’m not aware of any real downside to doing it that way.
Hope this helps.