1. What characterstics does the clock must have?
From CYBL10X7X Family Datasheet
2. In that case should XTAL24O pin be connected to ground?
Looking at TRM there are no recommendations, suggest you file a CASE -
“Create a Case”
3. What necessary termination does this clock input needed at PRoC?
None, although you might consider 10 - 30 ohms in series to the clkin pin to
dampen ringing. Normal practice on high speed clocks.