Could Caesar Clown have controlled Smoker?
The powers of Gasu Gasu no Mi would certainly allow Caesar Clown to control Smoker when he transforms into smoke using Moku Moku no Mi's powers, provided the smoke is within Caesar's range. 

However, Smoker would not be totally helpless if they fought one-on-one since he can fight without using his Moku Moku no Mi powers at all. He can use Haki, and carries the Kairoseki Jitte, both of which would be effective against Caesar.