Hi,
What Binding do you use for it?
I’m using Modbus Binding, my PLC is a bridge: Modbus over RS485 to Modbus over Ethernet, but you can use any Modbus converter or instead of RS485 you can use the arduino ethernet module.
How many IO ports + other all together on how many stains/busses?
im using it for sensors there are 5 arduinos, but i can’t see any problem to add more
Exactly what kind of devices?
What do you mean?
What firmware is uploaded to the Ardu boards?
There in no such a thing like firmware in arduino. Sketches are written in C/C++ like language
Is the Arduino ONLY a gateway, or can you use the IO + Analog ports on it too?
Every arduino is a Modbus Slave and you have acces to all goods on board 
What speed is it working? Can you long-press / dimmering / double click?
115200, all stuff like short/long/double etc. clicks, you can handle by arduino.