There're 12 Volt 5-6.5 A SMPSes availabe in market but the need is the lower 6-9V on such rate, so can we just lower the R parallel to the last C on output or it's too risky to safety?
No.
You would need to change the resistors in the feedback circuit, so it regulates to a lower voltage. How you do that depends on the design of the specific PSU.