Does it include the needed screws?

No screws included, just fan blowers.

would this work as replacement for stock cr10 part cooler fan ?

I used one on my monoprice select v2.1 and it works great. I of course had to print a new cooling duct and mount for it, but the fans I got are quiet and work soooo much better than the stock fans.

Im operating at 6.0 to 8.4 volts, I ordered the 5v is that going to work well or cancel for the 12v?

I don't agree with running it at 6-8.4V. From the manuf spec. it will draw 250mA at 5V e.g. dissipate 1.25 Watt for the USB version. The other is 0.4 A meaning 2 Watt. Increasing the supply to 6V will proportionally increase the current, thus increase power consumption and heat dissipation. The internal wire is calculated for 250mA respectively 400mA current. Stop reading if this is too much technical mumbo jumbo. If a higher current flows through the wire, chances are, the thin enamel coating will melt, thus shorting the winding out. In general the manufacturer calculate some safety margin into the wire gauge, but very tightly (less copper is more profit). I would not trust the wire enamel, once one turn is melted it will quickly take the rest with it. If around, it is not much of a problem, since a electrical odor will signal the demise of the fan. If away, not so good. It can potentially cause a fire. Also the driving circuit may suffer! The best thing to do is to put a 5 Volt regulator such as a LM7805 (1A) in line. I would also put a heat sink on, since there is in the worst case, still about 1 Watt to be dissipated at the regulator. Filtering capacitor are probably not needed. A around 14-15 Ohm 5 Watt resistor in line would, so la la, do the same except the fan would spin slower at 6V input. I am not a friend of this kind of patch work, besides a 14/15 OHM 5 W resistor may not be easy to come by. Good luck!