You can network Gen 2/3 and Gen 1 units. However, you cannot share Maps. You can share transducers and waypoints. The 5 can be used for single screen SONAR if you want, but it will be substantially slower. The Same transducers work for all units. You should update the transducer for Structure Scan to the newer version, though.
Hope this helps as a starting point.
2015 Skeeter FX 21
...and down goes PhotoBucket with the new Terms of Service... I predict they will be out of business or begging for forgiveness inside a month. 6/30/2017