Does doubling counters trigger Hardened Scales?
When the card says to "double the number of +1/+1 counters" on a creature, it means that you should place on to that creature as many +1/+1 counters as are currently on it. This means that Hardened Scales applies, so Setessan Oathsworn will end up with 7 +1/+1 counters on it.

Unfortunately, I can't find a rules reference to back this up, but the basic idea is that after Solidarity of Heroes resolves, there are more counters on the creature than before it resolved, and the only way that could have happened (from the point of view of the game rules) is if you put counters on it. There is no other applicable action that has that result.