1 -1/. which size and/or brand of external pendrive would work?
2/. what is maximum compatible micro sd card i can add?
I believe only upto 64GB is supported but I'm not 100% on that.
All (quality) brands should work.
SanDisk, Sony, Kingston etc even produce specific flash drives which have both a full sized USB and a micro USB.
The Samsung site states that is supports upto 32GB MicroSD, it doesn't mention Class. I would try a class 10.
One other thing, you said you're using a micro usb adapter. Is that a OTG cable or a standard adapter?