[Mark Allred]

Would it be terrible to fill the tank up from the tap using warm water? So that would be softened hot water mixed with unsoftened cold water, right?

I did a ton of internet "research" on this last August, as we were setting up a planted Angelfish tank for a friend with a water softener.

The consensus ended up being using softened water for an aquarium wasn't an issue. So....
For 8 months now, she is doing 50% weekly water changes using water run through an ion exchange resin system.
She does her WCs from her washroom with a Python, both hot and cold are soft.
Plants and fish are all doing very well.