OK - just remember that if you start fitting wider bearings, you are moving the centres of the bearing tracks closer together. For the one nearest the pinion head, it's moving the support for the pinion further from the point that the load is applied. I don't really see why you think the pinion 'wallows around' with 2 good bearings, or how adding a third midway between the two existing ones is going to improve matters. I've never seen any indication that there is a problem, provided the bearing closest to the gear end of the pinion is a 6004, not a 16004. (I did once have a 16004 pinion bearing fail on an early engine - it destroyed both 16004 bearings, pinion and took a couple of teeth off the crownwheel.)
If you look closely at the ratings of the 63004-2RS and 6004 bearings, you will see the static and dynamic loadings are pretty much the same, and the internal dimensions are the same - apart from the greater width, which is to allow the rubber seals to be fitted.
If you are talking about a hobby bike which covers only a few miles a year, let me reassure you. Around 1970, I rebuilt a Mk2 LD150, fitting new bearings (a 6004 and a 16004) to the pinion group. The same scooter is now sitting in my shed awaiting restoration. The engine has not been removed from the frame since 1970, and the pinion and rear axle have not been disturbed or failed since I assembled them - over 17,000 miles ago!
Unless you plan to do something radical to the engine to produce some serious power, I wouldn't waste time and effort to squeeze an extra bearing in the pinion group. However, if you do manage to produce some serious power, you'll have other more important things to worry about - like crankshaft tapers, torsion driveshafts & rear axle shafts to name just three....
If you still need a pair of the light cone washers and spacer washer, just PM me your address and I'll put a good used set in the post to you.