You can use the metal speedo drive gears on the snap ring boxes - even though they weren't designed for use or supplied that way. The same can't be said for using plastic gears on the locknut boxes - that's not an option as they'll just crush.
To use the metal gear you will need to make use of the 0.600" thick spacer (not in my photo) found between the bearing and speedo gear on the locknut gearbox. It'll need shortening slightly.
The plastic gears have a groove for the ball bearing to stop the gear spinning, however it isn't a full length groove so the gear can only be pushed up the mainshaft so far and is held in place by the circlip at the other end. The metal gears also have a groove but it is a full length one. You need something to keep the gear in place on a snap-ring box. Hence modifying the 0.600" spacer. This will sit between the circlip that holds the rear mainshaft bearing on and the speedo drive gear which itself has a circlip behind it.
The plastic gears come in two designs - the longer of the two are usually looser on the shaft than the shorter ones. I usually fit with bearing compound just to stop it moving around. If you were to use a metal gear on snap-ring box I would take this approach too.
Blue gears are 7 tooth, green gears are 6 tooth. There is a 7 tooth one that looks green but isn't green when you see it against the other green one! You can just make this out in my photo.
White driven gears nearly always come out mangled so I don't think they mesh very well with the 6 start gear they are normally meshed with. They're usually only found on standard Anglias so I doubt you'll need one.
HTH.