I've tried the most reason firmware version 1.20beta8+1 and i still get errors, when trying to use the mix feature M567.
I've also tried to enable with the M568 command, but it's obsolete now.

In the GCode i sent this command:
M567 P0 E0.951:0.033:0.020

And here's the reply in the console:
"Error: GCodes: Attempt to read a GCode float array that is too long: M567 P0 E0.951:0.033:0.020
Setting mix ratios - wrong number of E drives: M567 P0 E0.951:0.033:0.020"

To my best knowledge i've setup the tools correctly as you can see by the snip from my config.g file:

; Drives
M569 P0 S1 ; Drive 0 goes forwards
M569 P1 S0 ; Drive 1 goes backwards
M569 P2 S1 ; Drive 2 goes forwards
M569 P3 S1 ; Drive 3 goes forwards
M569 P4 S1 ; Drive 4 goes forwards
M569 P5 S1 ; Drive 5 goes forwards
M584 X0 Y2 Z1:8:9 E3:4:5 ; Apply custom drive mapping
M350 X16 Y16 Z16:16:16 E16:16:16 I1 ; Configure microstepping without interpolation
M92 X80 Y100 Z400 E95:95:95 ; Set steps per mm
M566 X900 Y900 Z12 E120:120:120 ; Set maximum instantaneous speed changes (mm/min)
M203 X12000 Y12000 Z3000 E300:300:300 ; Set maximum speeds (mm/min)
M201 X2500 Y2500 Z250 E1200:1200:1200 ; Set accelerations (mm/s^2)
M906 X700 Y1600 Z700:500:700 E2000:2000:2000 I30 ; Set motor currents (mA) and motor idle factor in per cent
M84 S30 ; Set idle timeout

; Tools
M563 P0 D0 H1 ; Define tool 0
G10 P0 X0 Y0 Z0 ; Set tool 0 axis offsets
G10 P0 R0 S0 ; Set initial tool 0 active and standby temperatures to 0C
M563 P1 D1 H1 ; Define tool 1
G10 P1 X0 Y0 Z0 ; Set tool 1 axis offsets
G10 P1 R0 S0 ; Set initial tool 1 active and standby temperatures to 0C
M563 P2 D2 H1 ; Define tool 2
G10 P2 X0 Y0 Z0 ; Set tool 2 axis offsets
G10 P2 R0 S0 ; Set initial tool 2 active and standby temperatures to 0C