So I think it will work well to mix saltwater, but a couple ideas to consider.
If I am looking correctly, you will gravity feed from the top RO to the lower container. You then will adjust the valves to recirculate the water in the lower bin to mix salt water.
There is a gravity feed for salt water and a separate one for fresh RO. It might be the angle of the photo, but for your saltwater feed looks low to the ground. If not, then looks good. For the freshwater you might consider a barbed fitting and tubing, which might be a little more flexible but it will work as is.
Another thing to consider is, If you want to use the pump as opposed to gravity for getting your saltwater you could also consider adding a tee and another valve above the pump outlet.
Looks good!