Are you talking about the original Blackarrow Pi image? And I'm fairly certain that the username/password root/pi is the right combo for default settings. If you changed the password via the passwd change for the root account then it will be whatever you set it to. If you've lost the password your best option is to start from scratch by re-imaging your SD card. ((The other possibility is that you aren't sshing to the right device)). Try using a client like PuTTY to connect to the PI.

I recommend backing up the following files for a quick restore if you ever need to reimage the SD card(winscp is free and work great):

/opt/bitfury/best.cnf
/opt/bitfury/proxy_pools.conf
/opt/bitfury/chainminer/jobconnect.cpp
/run/shm/.stat.log

As for other devices interfacing with the controller; afaik the pi GPIO is the only way to interface with the controller board and I'm assuming all the existing code is written specifically for it.