Hi Emelios,
You can get the ammeter to work correctly by just swopping over the connections on it
+ or - earth can be chosen which ever way you like, the bike does not care, after you polarise the dynamo and the regulator is the one to suit
The best solution for earthing is to run an earthing wires from ALL the components to a centeral point
(Battery earth bolt??) run a wire from the headlamp, taillight, dynamo, regulator and battery to this point *ex
*
Hope this helps
John OR