For mixer, you cannot have cold water from main and hot water from water tank (through storage water heater) as both water pressure differs, won't work as intended even with check valve.

You can Google 'Hot cold water pressure mixer different', there's quite a lot of information.

I installed quite a few sites, both the hot and cold water pressure are different, you can't really control the mixing properly with the mixer as the source with higher pressure will limit the other source.