Hi,
I recently had a friend who got a SD Wifi Card for an XDA, his worked fine and has had no issues.
1. I checked the Mio and as far as I can tell it is SDIO ready, so in theory any SDIO (SD) Wifi card should work, I say should as I can't be 100% sure.
2.I think cus more devices have WiFI built in these cards seem hard to find, although did find this on Amazon
http://www.amazon.co.uk/SanDisk-256M.../dp/B000300RXW ebay might be a better place to get a better price from someone who is upgrading.
3.As far as I can tell you will be able to use any apps which you can run on the Mio and that can see the internet.
4.Unless you use a card like the one off of amazon you would not be able to use any other SD cards, the WiFi card will take up the only SD slot on your device. Also I don't think the capacity of the combined cards reached above 256mb.
Hope this helps.
Steeve