HDS GEN3 Bonus Units/Installation

Guys, 

We were given a couple units from a close friend now that he's upgrading his boat and need suggestions on the best way to set them up, transducers, etc. We currently have an Astro 2000xl. We bought it used with no sonar. Because we are on a budget, we have been trying to upgrade when we can. Started with a 9" Elite-Ti2 at the console. This April, I added a 9" Elite FS up on the trolling motor. We have now been given two (2) 12" HDS GEN3 units, structure scan(?) black box and the Totalscan transducer. Both transom and trolling motor have 3-in-1 transducers I believe. 

Would like ideas for location of units, function and networking. We almost exclusively bass fish, don't really crappie fish and my dad wants to be able to chase hybrids around from time to time. Right now we are thinking one 12" back at the console with another 12" up front with the EliteFS (to run active target at some point). Is there a benefit running HDS over the EliteTi2 at the console for side/down imaging? Will the transducers I have already installed for the Ti2 and EliteFS work with the GEN3's or will I have to install the Totalscan? Will we be able to network units together? What are the strengths or major blindspots of the GEN3's? 

Any help is much appreciated.