Ride: 07 Strada GLS

Need advice on how to install my sound setup to get optimal results..

Recently purchased the following:

Front Stage: Polk MOMO MMC6500 6-1/2 inch Seps - Max 125W RMS
Rear Stage: Polk MOMO MMC650 6-1/2 inch Coaxials - Max 80W RMS

Amplifier: Hifonics Zeus ZXi8006 5 channel Amplifier

Specs: 740-Watts A/B-Class

4 x 110-Watts + 300-Watts x 1-ch RMS * 2-Ohms
4 x 55-Watts + 200-Watts x 1-ch RMS * 4-Ohms
2 x 220-Watts + 200-Watts x 1-ch RMS * 4-Ohms (Bridged: 3 channels)

Hindi ko maintindihan kung ano yung Ohm.. Do i simply choose kung anong combination ang gusto ko sa Amplifier between the 3 options? If so, what is the best option using what I have? Do i go for 2 ohm or 4 ohm connection? and AFAIK, both my speaker sets are 4ohms so dapat ba 4 ohms? Or can i run it in 2 ohms para louder? Or do i need another amp? Kulang ang space sa Strada that is why i went for a single 5 channel amp. Kulang pa ba ito?
Also do i need to take into account the RMS of my Speakers? Ok lang ba ma underpower / overpower? Pardon the multiple questions.

Also I dont have a sub yet but planning to buy one soon. Any recommendations on what brand? Shallow / Slim mount lang ang pwede coz I'm installing it behind the rear seat. What is the best Shallow mount sub in the market right now? and again, should i go for a 2 ohm or 4 ohm sub?

Bottomline is I want to get optimal results from my setup. SQ lang naman habol ko.. I hope someone here can help.

Peace!