Solidarity of Heroes

Solidarity of Heroes can target any creatures, not just ones with +1/+1 counters on them. Notably, heroic abilities of any target creatures that put +1/+1 counters on that creature will resolve before Solidarity of Heroes.

If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen. If one or more of its targets are legal when it tries to resolve, the spell will resolve and affect only those legal targets. It will have no effect on any illegal targets.

You choose how many targets each spell with a strive ability has and what those targets are as you cast it. It's legal to cast such a spell with no targets, although this is rarely a good idea. You can't choose the same target more than once for a single strive spell.