Got to agree with current opinions on Austria being the best ally, despite tensions over Galicia. Pragmatism/realism will be a major fuel
Prussia and Russia arent going to treat as a proper ally, more of a buffer zone that can be used and thrown away once its no longer useful in the geopolitical game. Austria is at the stage where it needs an ally that will rely on them and not the other way round. The PLC would help to deal with Prussian aggression and protective barrier against Russian invasions and influence in the Balkans.
Despite that Austria likely wouldn't be the only ally the PLC is going to go for, possibly Sweden, the Ottomans, the South German states, France. Another state to counter balance one or the both of the others so they aren't the sole/main battle ground.
Edit: As for the population some will have problems with any alliance, no option will make everyone happy because all have wronged/humiliated the PLC, though if I remember right Austria is the lowest offender in recent memory