What they can do when connected together is still in the public testing phase.
Stuff that was assumed would work between Helix and XX9 models doesn't seem to be functioning for some users.
For sure a 999ci HD SI Combo and a 999ci HD SI Combo will share all data including sonar, GPS, and Ethernet accessories.
The 900 series is among the discontinued models and the Helix models are replacing all the core units.
The Helix 9 SI GPS comes with the Compact SI transducer like the 899ci HD SI Combo does--lower price point out of the box. BUT both of those have the 800 kHz Imaging capabillity to fully utilize the HDSI transducer that is standard with the 999ci HD SI Combo.
It costs $90-$100 for exchange of the transducer through Humminbird Customer Service. That includes the transducer cost difference, the shipping to you and you shipping the Compact SI transducer to Humminbird.
The Helix models are too new to have much user experience to share, especially the Helix 9 and 10 series. The Helix 12 series is due out sometime in Feb.