Set constraint for timequest

1.

Besides constraining the clock as usual, you don't have to set more constraints.

However, this will result in a over-constrained circuit: TQ will ignore the clock enable and analyze the circuit as if the clock is always enabled.

If the circuit can meet your required timmings in thisway and you're happy with the area it's taking, then fine.

If not, you'll have to relax the timing constraints for the clock-enable driven circuit by setting mutli-cycle constraints.

2. No.

3. Not familiar with the NCO, but are you using any of it's outputs as a clock signal?