RMS power ratings are not the exact number required. RMS means Root Mean Square. It is the "point of efficiency". Speakers can usually take more power. Some speakers can take a lot more power. You are just adding more power and not getting an equal return on your decibel/volume. Its a curve that is spiking after RMS for power usage, but giving very little return on any improved performance. (EX. If 150watts is your given RMS, and it gives you 95 decibels, 200 watts might only give you 96 or 97). It's generally doable, it's just not usually worth it.
When you start factoring in how amperage draw, and upgrading alternators, batteries, wiring, electrical overall, you can step it up to a level that is not worth it. That's just not a concern for what you are wanting to do. It's really a question of whether the speakers can handle it. Any number given to you for peak power is rated at full volume (when your gain is set properly) for quick bursts. If you don't listen to your music at full volume, then you would be okay. I searched and those 6x8s are 75w RMS and 225w peak. I would feel comfortable running those at 150w.
Running two speakers on one channel will either make them run on the same side, or make them run in mono. I would avoid that for anything other than subs.
I would just use a 150w x 4 amp, and you should be fine.
Are the 6x9s in a decklid, and the 6x8s in the door? You could see if getting an adapter (if you even needed one) will work, see if you can trim the sheet metal or whatever is there around the mount, and get a second set of matching 6x9s to fit where the 6x8s are going to go. (You already have one to test fit). If it does, you can run the 6x8s and have a backup plan.
If it's in the budget and you considered one. Now would be the time to look at 5 ch amps if you wanted a subwoofer. A 5ch amp could be right there in price where it could be cost effective with a 4ch amp, when adding a second sub amp isn't. Not buying a second amp, or not having to run extra wire, or distribution blocks can save a little on the overall setup cost.
