Out of all the Corleone men, Sonny would've been the most likely to be able to pull off such a coup. Sonny was Sollozzo's potential "in" with the Corleone family and its resources, so he would've been safe from a plot on his life...at least temporarily. Luca Brasi had no such safety going in.

The problems that I can see with Sonny wanting to wet his beak in the drugs trade on his own are twofold: firstly, what is Sonny going to bring to the table? He wouldn't have direct access to his father's resources, such as the political and legal contacts, financing, et al, especially if he's meant to be flying below Corleone radar as it were. Those were the things that Sollozzo wanted from the Corleones at the outset.

Secondly, his temper. I've no doubt that Sollozzo would've sought to test Sonny. Quite likely, Sollozzo would bring up the subject of needing Vito "out of the picture" so that Sonny could ascend the throne. I don't think Sonny would've been able to hold frame with talk of his father being whacked. He would lose his temper...and blow his cover. At best, he gets sent packing with no real intel. At worst, he gets sent packing to the bottom of the Hudson River by way of cement shoes.