KSC,
Welcome to the forum. Glad to hear you have had some great success with the 998.
To answer your questions:
1) is that sensible thought about scanning 100 meter depth with SI?
No, you will not be able to get 100 meters depth with the 262kHz. Max is going to be around 60 meters.
2) if it is possible ; are we able to use this transducer with Y cable, do they use the same input , i mean same cable ?
Yes, you can use it with the Y cable.
3) can we use this transducer with 998 standart transducer simultaneously for quadrobeam ?
Not sure what you mean about using "simultaneously for quadrobeam", but I think you are asking if you can use the 262kHz and 455kHz at the same time? The answer would be NO. The Standard SI (262/455) Transducer is just like the HDSI (455/800) Transducer. It just has different elements (frequencies) for the SI. You can only use one freq at a time on SI.
Hope that helps,
Robert