Since Tom was the contact between the Tattaglias and Solozzo for the drugs meeting, it wouldn't have been difficult for Sol to read Tom's reactions and figure out that Tom favored the deal. "My compliments" was, as you said, Sol buttering up Tom. It was no intelligence coup for Tom to know that the Tattaglias were backing Sol--he never would have gotten to see Vito without a major family vouching for him.

It's true that Vito missed seeing Barzini in the Solozzo deal. I've posted earlier that I believe Sol would have gone to Barzini first because he would have figured that Vito, already on top with rackets that were a lot safer than Sol's, would have nothing to gain from the drugs deal--whereas Barzini, younger and more ambitious than the other Dons, would. Barzini would have told Sol: "Your logic is good. But, you still need Corleone's politicians and judges. if Corleone hears that I'm your partner, he'll say no. Go to Tattaglia: he's a pimp, Corleone's not worried about him. But, I'll be your silent partner and I will have your back."