Let's calculate it. The vortex viper 4-16x60 hs-lr has a total of 75 MOA of internal available elevation. If you run a standard (zero cant) base, your zero will be about in the middle of the 75 moa. This will allow you about 37.5 moa up and 37.5 down.
If you run a 20 moa base, you decrease your bottom end by 20 moa, which puts you at 17.5 remaining in the down, and 57.5 up for elevation. I recommend not going less than 10 moa remaining for your down amount. IMO, you have enough room for running a 20 moa base. But, when the reticle is placed at the near bottom end of the scope, you decrease the amount of available windage. 17.5 moa should be enough, but if the scope is mounted right or left, you may run out of left and right adjustment.
In this rifle set-up for the 6.5 CM, you will need about 25 moa to get to 1K yards. So, even without the 20 moa base, there is still plenty of elevation available (37.5 moa) to reach out to 1K yards. So, is the 20 moa base needed, no. Can you still use a 20 moa base, yes, but with the added risk of the windage issue. I wouldn't run a 20 moa base with this set-up. The scope will have full function and full windage available without the 20 moa base.