Here is how I had my Oasis rigged.
http://www.bigwatersedge.com/bwevb/s...ead.php?t=7760
In hind sight, i wish I hadn't sold it, particularly at that price
I used Ram mount balls & rocket launchers for the rodholders. They make the same balls that will attach to the YakAttack rails. I liked that they would all lay flat for transport, and I could rig them up for use. i wouldn't overthink the 4" vs 6" YakAttack thing. One thing that you will want to think about, is rigging the yak up with at least 1 paddle on, because you will always have that on board, and you dont want it getting in the way of any accessories.
When fishing tandem I'd keep the bait tank in back, when solo I'd keep it up front for ballast to keep the bow down. I used the same fishfinder between 2 yaks, just like you are describing.
For the fish finder, find one in your budget and go from there.
Good choice in Yaks. You'll have fun.
Steve