2 Alpine Type-R 10"s (1000w each)
Kicker CXA1800.1 Mono Amp (1800w)
JL Audio C2-525 (60w)
JL Audio C2-690tx (125w)
Rockford Fosgate Punch P600x4 4 Chan (75x4) Amp

- Would this setup sound decent and do these subs sound good (don't want to crack my brand new windshield)?

- Also, would I need a new battery or alternator?
(Big Three is already going in)

- Can I mount the amps under the front seats?

- What gauge power and ground wire should I go with?

 

just commenting on subs and amp:
Assuming those type r's are 2ohm Dvc that should sound great! Since your amp is 1800@2ohm 900@4ohm. Looks like kicker recommends 0/1 gauge wire
https://pdf.crutchfieldonline.com/ImageBank/v20160530131500/Manuals/206/206CXA1800.PDF
good luck!

 

1800w is about 150amps of current at max power, so you will need the 0 gauge wire someone else noted above.

I'm not sure they would fit under the seats and have enough airflow for the amps to stay cool - its not a big space after all.

Have you considered buying a 5 channel amp to make installation easier?

 

you will also need to make sure your alternator can handle all that. I had a single 12" and would make my headlight dim with the bass. I switched to a kenwood 4 channel amp with 2 infinity kappa 4x6 120 watt rms @2ohm and 2 polk 6x9 100 watt rms @ 4ohm. ill be switching out the polk for some infinity kappa 6x9 that can handle 240 watt rms. the 6x9's put out some decent bass. the amp has built in filters so I just do low pass for the rear. if your inclined to do so you can also run an additional 4 speakers from your head unit.