Id like to keep them inside the cabin to stay out of the elements..Im buying marine grade components but why expose them to more than you have to!

Amp closer to speakers /subs= Less wire>less resistance>more efficient use of power. That being said, Its not pratical to run your amp wired directly to a power source with 6" of cable, the have your subs mounted inches away from amp outputs either. Mount amps in dry secure locations, and speakers /subs in practical areas with good 'imaging'( locations/line of sight to your ears). Use good quality RCA's, and the recommended ga size for power and ground cables on amps.. Those kicker amps will prob need a 4ga power cable. Check the specs, run a cable capable of keeping up with the amp' power demand. Thats prob the biggest mistake people make, running too small of cable on thier amp. a 2000 watt amp cant draw enough power thru a 10ga cable...

That's exactly where I am putting my amps. One of the previous owners put a 2 channel amp in there to run the cabin speakers...must have been a while ago because the amp being used is a 1980's era Kenwood. If that amp has managed to stay fully functional after sitting in the cabin forever, I don't have any qualms about the non-marine JL amps I am going to be putting in there. I'd be concerned if I was exposing the amps to salt air, but not a worry where I live.

The amp suggested gauge wire run is 8ga..would it be beneficial..detrimental..or not matter if I ran a heavier gauge say 6 or 4..and is 16 gauge speaker wire sufficient!?

Never hurts to run heavier gauge power and ground wire to the amp, except maybe the wallet.

16 gauge to the 6" speakers should be fine.

^THIS-Also, use 12ga-14ga for those subs. Generally, I use the biggest ga cable that will actually hook into the amp. If it will only accept 8ga cable, taking a 4ga and clipping half the wire strands to get it to fit into the amps' input terminals is a waste.
Good luck find amp istall kits, as you can find them with 20ft power cables, but every one I've seen/used only has a 3 ft ground cable. Not quite enough normally to find a ground in a boat. Go to Napa or Advance Auto parts, they normally have spools of different ga cable you can purchase by the ft.

Im a HUGE kicker fan. for this type of install i would run some 1/0 AUG of both power and a ground wire for the amp run them from the batteries to a distro block in the cabin dont skimp on the wire size it will cause your deck and amps to clip and shut off. i would run 4 aug from the distro blocks to the amps. with the speakers you choose a kicker 300.4 amp would be decently loud and nowdays crossovers are kinda overrated with the settings on the deck and amp unless you go have it professionally tuned i wouldnt get one. now beings its in the cabin a 300.4 car amp should be just as good as a 300.4 marine amp and probably a little cheaper. sub amp i would run larger in case you want to add more subs later. 750.1 or something of the nature. best quality sound will come from a 4 OHM setup. the lower the OHM rating the harder and less efficient the amps run.

remember with the backbone (IE AMPS) if you go to small you have to add more if you want to expand if you go bigger from the begining you can DETUNE your amps to handle the power of the speakers you have now. (IE turn the gain down on the amps)

ANY system over 500 watts should be grounded back to the battery. in a boat run both GROUND and POWER to the batterys or you are just asking for issues with equipment failure.

Coax speakers, 2 ways are decent and will sound good but if you want sound quality go with a component setup the clarity cant be beat from a component system to a coax system. plus with a componet system it will sound louder with the same amp tuned the exact same way than with coax speakers.

Really like your choice in the Sony head unit. I have a very similar but much older model. Sound quality is outstanding and outperforms Kenwood all around. I have always had good luck with Coustic amps, but it's been awhile since I have done any research.