To answer your Q's in order Bao:
when must the clutch be engaged? is there a minimum ERPM and what do you check once you engage it - RRPM, engine and rotor needles matching-?
Soon as the engine is started and you get oil pressure, clutch switch to engage.
What is the function of the CLUTCH warning light? does it ever illuminate in flite?
To tell the pilot that the clutch actuator motor is operating. It does from time to time illuminate in flight but only when the belts are being re-tensioned. If the light comes on and stays on for more than a set time (7 seconds I think, but I don't have a PoH in front of me) then you have to pull the CLUTCH CB, land and get it checked out.
Which means do you have to monitor the clutch?
The light. As above its on whenever the clutch actuator motor is running, either up or down. It can be heard running down POST shutdown its its not too noisy outside.
How is related to the rotor brake?
No connection at all*. The rotorbrake is a simple mechanism attached to the main rotor gearbox that is operated by pulling a chain above and behind the pilot after shutdown. The RB is two arms with 'brake pad' type material that when pulled contact the MRGB output yoke, which then slows down the M/R.
*They have one small connection. If the clutch switch is in the ENGAGE position and/or the rotorbrake is ON, then the starter in inhibited and you wont be able to start the engine
When do you disengage the clutch?
30 seconds after pulling the mixture to stop the engine. If you have new belts fitted to the A/C then there is a procedure to not disengage the clutch in order to stretch said new belts. But thats a bit of irrelevant info for you.
Hope that helps.