Motor - Very slow motors (300kv - 1000kv) are usually made for multirotors. Slow motors (1000kv - 1500kv) are used for 3D planes, slowfliers, or stuff like that. Faster motors (1500kv - 2500kv) are mostly for faster planes, racers, and that kind of planes. Very fast motors (2500kv - 5000kv) might be micro motors, if not, they are usually for EDF's (electric jets).
That's not necessarily wrong but it only holds true if you are using the same voltage battery for all applications. For a general use 11.1 vdc 3S battery then that's a good rule of thumb. But, since this is a larger heli question we're talking higher voltages.
Lipo cells are all 3.7 nominal voltage. Therefore your voltage is 3.7 x number of cells. 1S = 3.7, 2S = 7.4, 3S = 11.1, 4S = 14.4, 5S = 18.5 and 6S = 22.2
The KV rating has to be matched to your supply voltage and gearing to result in optimal prop or rotor RPM. Say that you are trying to get a prop speed of 10,000 rpm at full throttle.
1s 3.7v x 2700 kv motor = 9,990 rpm
2s 7.4 x 1350 kv motor = 9,990 rpm
3s 11.1 x 900 kv motor = 9,990 rpm
4s 14.4 x 700 kv motor = 10,080 rpm
5s 18.5 x 550 kv motor = 10,175 rpm
6s 22.2 x 450 kv motor = 9,990 rpm.
The difference is that the lower KV motors are usually larger motors meant for larger heavier planes. They are doing much more work so they need the higher voltage to operate correctly but they spin slower per volt so that you don't overspeed your props. You will find that the rules are pretty much the same for all airplane props.
Now with helicopters you are talking much lower head speed, usually less than 3000 rpm for a small heli and closer to 2000 for a mid performace 550. So with a blade 450X your setup is as follows.
11.1 kv battery x 4200 kv motor = 46,620 motor rpm.
10 tooth pinion and 142 tooth main gear = 14.2:1 ratio. So, 46,620/14.2 = 3,283 max rotor speed on a Blade 450X.
The standard 450 has a 3800 kv motor. Same calculations. 11.1 volts x 3800 = 42,180 motor rpm / 14.2:1 gear ratio = 2970 max head speed on standard blade 450.
Apply this to the recommended setup for the 550.
6s 22.2 volts x 1400 kv motor = 31,080 motor speed.
16 tooth pinion and 170 tooth main gear = 10.625:1 gear ratio.
31,080 motor speed / 10.625 gear ratio = 2925 max rotor speed. Very acceptable.
If you used the same motor but only 5s batteries it would go as follows.
18.5 1400 = 25,900 motor speed / 10.625 gear ratio = 2,437 max rotor speed. You would notice a severe lack of performance compared to a 6s setup. You would need a 1680 kv motor to get the same rotor speed. That changes your amp draw though because you are using less voltage. Less voltage means more amps to get the same amount of work done.
So, you can see that general recommendations don't really hold true for helicopters because we are using such deep gear reductions and most brushless planes are direct drive.