After trying out, reading a lot and even talking to Android App developers this is what I know at the moment:
Only Android 6 and later offers the MIDI over USB Mode. (And it seems that you then have only 1 MIDI Port, maybe someone can confirm or correct this?)
The Galaxy does not support Android 6 and theres is no info if and when it might be.
If an app has drivers for devices like Midi Interfaces built in - no problems. This is the case for Lemur on iOS.
I bought the ik multimedia MIDI iRig2 including the offical OTG Adapter but Lemur does not recognise it because there is no driver. According to an App developer its not possible to install drivers (like on a computer) which then can be used by the apps on Android.. At the moment I'm running Lemurs Midi Connection over WiFI which is a pain. It's slow and buggy and i have to restart Lemur 10 times a day because the Midi Connection is lost.
There are rumours about another 18" tablet w/ Android 6 which might be released in June 2017. Otherwise going to an ipad is another option.
Did not think its this complicated