Fancy Seats,
Personally I would use the big HDSi transducer, but than I seem to spend more time fooling around with the sonar versus fishing… You need to decide if having Si sonar up front will be helpful to you or not and whether it is worth the extra money that the HDSi transducer will cost you.

No, there is only one transducer connection on the back of the 998c Si unit.

Yes the TS3 Transducer Switch will work for all of the sonar.