Yes, Sol was remarkably truthful to Tom and Michael--with one exception:

He tells Tom, "Sonny was hot for my deal, right? And you knew it's the right thing to do." RIGHT.

"The Tattaglias are behind me with all of their men. The other families will go along with anything that prevents an all-out war." TRUE.

He tells Michael: "this is terrible --it's not the way I wanted things to go at all. It should've never happened.." TRUE. He intended to make a deal with Vito, and then intended to kill Vito.

"What guarantees [about not bothering Vito again] can I give you? I'm the hunted one. You think too much of me, kid...I'm not that clever. TRUE. He can't give Michael a guarantee because he wants Vito dead.

(Earlier, in the car): "I swear on my children he [Vito won't be bothered again]." FALSE.