Seachem's fertilizers are the same as dry fertilizers except they have water. So yes you can use them.
Instead of using all K2SO4 for your solution use the Flourish potassium.
Warning: I included my math lol
Flourish Potassium has 48mg/ml of potassium
One mg of K2SO4 contains 0.448736 mg of potassium.
So lets say you want 1 liter of PPS solution with 1ml dosing 10 gallons...
According to the PPS-Pro mixture located here
you add 59 grams of K2SO4 to 1 liter.
Plug the numbers in,
59,000 (59 grams) * 0.448736 (1mg of K2SO4) = 26,475.424 mg of potassium
So let's say you have 139ml of Flourish potassium left.
139 * 48 = 6,672mg of potassium
That means you need to add 19803.424 mg more of potassium via K2SO4.
19803.424 / 0.448736 = 44,131.57 mg of K2SO4
So you simply add 139ml of Flourish Potassium and 44 grams of K2SO4 to the one liter bottle.
As far as the iron is concerned that's really not feasible since Plantex has iron in the mix. You can however dose both. The iron in Plantex and Flourish are different. The iron in Flourish is gluconate which means it does not last as long in water but is more available to plants. So the addition of both will provide a better source than Plantex alone.
One PPS dose of Plantex would provide 0.14 ppm of iron according to the link above. Adding 1/3 of that via Flourish iron would be a good balance.
Plantex has 0.0653mg of iron per mg.
Flourish iron has 10mg/ml of iron.
80,000 (80 grams) * 0.0653mg = 5,224 mg of iron
5,224 / 3 (to get one third) = 1741.3 mg of iron
So we need to add 1743.3 mg of iron via Flourish to our 1 liter bottle.
1743.3 / 10 = 174.33 ml of Flourish iron
Your problem is much easier. Just use the Flourish comprehensive at a rate of 1.2 ml per 10 gallons instead of the PPS-Pro micro mix.