Is that too much light?

Now, that's just crazy talk.

I like the 6 n 2 setup.
You can go from 3:1 R:B to 6:1 with the flip of a switch.
Though a dimmer would give the control that you want.
I like the centered blue array as well.
Blue triggers phototropism and we want the terminal bud to seek the center of the light.
2 fans should be jus' fine.
Depends on how hard you drive the leds.

Once they are mounted on the sink, you can light 'em up with the variable supply, and see how a passive sink handles the heat.
If it stays under 40 degrees C. you don't really need a fan.
Between 40 and 50 C. in the first hour?
Add a thermal switch in series with the fan to conserve power.
Over 50 degrees in a the first hour of burnin?
Hardwire both fans.

On that PWM.
Prolly the same one that I fried.
Can't blame the device though.
I pushed my luck with a fraction of a volt over the max rating.

(Li'l tiny Zards make big dumb mistakes alla time)

Nobody's prefect
Weeze