What size Diamondback nozzle should I use on a SV06 Plus? [
diamondbacknozzles.com
None, I fear 
The volcano compatible ones have too short head, I believe. Look at this ruby tipped volcano nozzle I have, compared to an original SV07 (same as SV06plus in that):
When I find the time I will try an “ordinary” volcano nozzle with my SV07 (probably have to remount the distance sensor) - if that works I will tell 
If you adjust your cooling fan and maybe the inductive sensor you should be able to use regular volcano nozzle
Thank you for your input
as promised here are the results of my tests.
tl;dr: it works <evil scientist’s laughter>
In detail: I had to replace the fan outlet as mentioned in (1), and also the probe holder from (2) (that thing is a godsend). Then all the equipment had a nice distance to the nozzle tip (after cutting the rubber socket. Don’t cut too much! And: For standard nozzles you want to keep a original nozzle - (3)).
Aligning the printer bed was an obvious necessity. And then: Well, my ruby nozzle did it
(1) Printables
(2) Printables
(3) Sovol 3D Printer Hotend Silicone Sock, Silicone Cover for SV06/SV06 Plus/SV07 (5 pcs)
0.8mm, IIRC
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they, of course, work fine