1.) Looking into now, Just going with a certain brand won't solve the problem, brands use whatever chipset they feel like, and the chipset is what matters. You could look around wikidevi to find out chipset/adapter names, then google to see if anyone's posted on the net that they work with MAC changing or not. I'm currently waiting on an RTL8192CU but might also get an RTL8188CUS since there are tutorials for that as well. Those two have relatively cheap options but other than that I'm not sure. I CAN tell you that my atheros TL-WN822N V2 doesn't work, but their v3 is an RTL8192CU, though probably more expensive than adafruit's options.
RTL8188SU does work with mac changing in LINUX but if you do it on windows it won't connect to the internet, and hostapd on linux doesn't work with it.
and my alfa AWUS036NH doesn't want to get a changed mac, so though it might work with hostapd, I didn't bother... Speaking of, since it doesn't work and another user's reported that an RT3070 does work, I'm wondering if maybe the ones that connect directly to the computers USB port will work, while the ones that have a cord between them dont/won't... I'll have to see if my other atheros card works sometime.
2.) Yes. You can use any mac for streetpassing, as long as your SSID is set to attwifi or whichever NZone SSID you want to use for it, that your 3DS picks up.