Can the K2SO4 be mixed with the above?
Yes it can. Just don't mix the micro nutrients with macros.
Also my EI is not 100% daily, as I do Macro one day then Micro Next, so when working out how much K2SO4 to add to 500ml, do I need be calculating for The Estimative Index rather than daily?
Honestly, as long as your'e staying within the ranges I listed your'e good. So you could do either. That's one of the beauties of the EI method. There is a lot of wiggle room. Often I see people trying to calculate this to death. Just get in the range and your'e all set.
"To reach your target of 7.5 ppm K you will need to add 41.784 g K2SO4 to your 500.0 mL dosing container. Add 18.0 mL of that mix to your 90.0 L aquarium to yield"
I would be dosing it 3 times a week so that method would be correct?
Yes it's fine 3 x 7.5 = 22.5ppm in a week. The range is 10-30ppm. See the wiggle room? lol
Either way which ever option I go for, when I dose this with the Macro/Micro mix, they wont cancel each other out?
Just mix the macro and micro seperate. Dose on alternating days. Technically mixing the two can cause precipitation. I've never seen that happen and I dose both on the same day. I just make sure each is mixed into the water well before I dose the next.