If my Maushold ex with the "Solidarity" Ability is active and damaged by an attack and I have an evolved Maushold on my Bench, do I count the basic Tandemaus underneath the evolved Maushold when adding damage counters for "Solidarity", or do I just count the top Evolution?
No, you only look at the most evolved stage of a Pokemon in play. The previous stages are now part of that Pokemon.
Source: Paradox Rift FAQ; TPCi Rules Team (2023-11-02)