Mixing can primariliy be a function of solubility - it sounds like all of your processes are water based? If so, then mixing might be boiled down to solubility, and amount of solute (what is being disolved).
Here is an easy equation: ranking = (solubility) / (amount to dissolve).
The higher the number, the easier it is to dissolve. This is because solubility is high, or amount to dissolve is low.
You could take the top 3 products, and test them in the lab (quick and cheap). Then you could take the top 1 product and test it in the field with a full process validation.
This of course assumes that the mixing equipment is the same. A change in mixing, might require a separate process validation (they are different processes and should be validated under a different process validation).
Overall make sure that all factors are accounted for. Affect of temperature on mixing, chemical reactions, etc.
Viscosity is one of many primary things to look at. Fill weights, fill containers, etc, all should be a part of filling process validation. There might be some similar products which can be bracketed, but likely there are a bunch of presentations which are different enough that 1 single bracket approach won’t work.