“Would the viscosity increase or decrease if i use Cetyl Alcohol instead of cetearyl alcohol? ” - I don’t know. You have to run the experiment in your system and find the answer.
But the real answer is, the system is too complicated to simply say whether it will increase or decrease the viscosity. There are factors of solubility, polarity, ingredient purity not to mention particle size, forces involved during mixing, etc. Unless you create a phase diagram you have no idea of what the crystalline structure will be.
All the stuff written about it from makingskincare is just theoretical musings about some imagined idealized formula.
People generally use Cetearyl Alcohol because having a blend of fatty alcohols leads to a more stable formula than single fatty alcohols. But then again, it depends on the system.