sdram use different clk with other ip in nios

question1:

Qsys automatically inserts a generic clock crossing bridge between devices operated at different clock frequency. For improved performance you may want to add an explicit clock crossing bridge component and configure it according to your needs.

Anyway, I believe in your case the frequency splitting is not strictly necessary. What device are you targeting? Cyclone III and later devices can easily run a Nios system at 100MHz, so you'd rather setup everything for this frequency and avoid the delays and latencies involved with crossing clock domains.

The conservative 50MHz is typical in reference designs: it helps you to easily meet timing constraints, at the expense of performance.

question2:

The answer is definitely Yes