They knew if they took Sonny out of the picture, then they would defeat the Corleone family as they thought Hagen would be inadequate since he is not even sicilian and they didn't consider anyone else in the family to be able to wage war against them. They underestimated Vito's strength and health, as Vito hearing the news pulled himself together so he can save the family from certain destruction from the enemies.
Sollozzo is not really a Tattaglia as he's allied with them as business partners, since the Tattaglias would be doing the operation procedure in the drug business, Sollozzo was setting up.