IF your Stratux is built and configured to receive UAT, AND your are flying in a country where UAT is available (US, CA, Denmark I think?), THEN use GDL90. ELSE use NMEA.
Reason: GDL90 uses UDP as its network protocol, which is a good thing in theory, but with UDP it can happen that packets are dropped, delayed or even received multiple times. It's just part of the protocol and to be expected. Sadly, SD is rather intolerant when it comes to such errors.
Additionally, NMEA has proper support for the bearingless circles, yes. You can not use both at the same time.
This depends on what hardware your Stratux contains and how it is connected. No idea how Dross does it. I assume you don't have a fan control module in there, so you can either connect the fan to 3.3v or 5v. I prefer 3v to have it a bit more quiet. Airflow is good enough either way (well, depends on the fan of course, but full-power is certainly not needed). Google "Raspberry Pi Pins" to see how/where to connect.