high power rgb led current matching
Assuming the three LEDs (R,G &amp; B) have individually accessible leads in the package they are housed, I'll offer the following advice. This means the package should have 6 leads so you will have complete flexibility in how you hook them up.

Driving the R,G,B LEDs in one unit in parallel will not work because the red LED, which has a lower Vf than the G or B LEDs, will clamp the output voltage of the constant current source to  its own Vf (5.4 to 6.6 volts). You will only get red light from the unit because the current from the CC supply will only pass thru the red LED.

Driving multiple RGB LED units in parallel will also result in creating only red light. However, you will find that each LED unit will produce different amounts of red light. One red LED will look bright, another relatively dim. This is due to variations in the Vf of the individual red LEDs. This piece-to-piece variation in Vf is probably included in the "5.4 - 6.6V" spec you state. 

Also, the red LED, which gets the most current because it happens to have the lowest Vf of the group you are connecting in parallel, may overheat because it is "stealing" current you intended to flow thru all N of the red LEDs.

You will solve all these problems by connecting the LEDs together in a series string. So the forward voltage drop of one RGB unit will be nominally 24.3 volts (6 + 9 + 9.3 volts). Depending on the actual Vf of the individual LEDs you happen to connect in series, and how much current the CC supply actually produces, this total voltage drop could be as  high as 25.5 volts (6.6 + 9.3 + 9.6). So the CC supply must have enough "voltage compliance" to reach these voltage levels. 

To run multiple RGB units you should string all of the LEDs together in series for the same reasons I gave above for one RGB unit. Therefore, your CC supply must be capable of producing 50.1 volts for two units, 75.6 volts for three units, etc. 

Your other option is to split the output of the CC supply in N streams. Each stream will supply the desired 300 mA current to each RGB  unit with its individual LEDs connected in series. Look up a Linear Tech LT3092 and study the example applications given in its data sheet. You can use several of these devices to split up the constant current supply's output into several nominally equal streams. 

By the way, if you don't get the proper proportions of current flowing thru each of the R,G &amp; B LEDs in a single unit correctly balanced, the unit will not produce "white" light. The produced light will be tinted red, green, or blue. In fact, you may find that the light will be quite noticeably reddish, greenish, or bluish. I suspect that the manufacturer is intending each LED to be driven at a different current level even though the nominal level is 300-330 mA total. You will know this right away when you hook up one unit's LEDs in series because they are forced to all experience the same current flow in a series connection. If this off-white condition is not acceptable you will need to regulate and adjust the current thru each individual R,G and B LED. One simplification might be to connect all red LEDs from multiple units in series, all green LEDs in a separate series and the same for the blue LEDs. Of course, you will need separate CC supplies for each of the color strings.

As you can see this can get pretty complicated, pretty quickly. If your spotlight is a one-off project, you can take some shortcuts depending on the actual LED units you have on-hand. If you are designing a mass produced product, you have some real tough design problems to overcome. 

To answer your other question: Yes the CC supply will adjust to the voltage required by the load (LED or LEDs) at the specific current being supplied. This is called the "compliance range" of the CC supply. In general, you want a CC supply to have a very wide compliance range. However, many simply constructed CC supplies have a very limited compliance range.