I don't know enough of the SB genetics - it is said to be copper based, but I get no coppers when crossing them to blue - only blackish blue and black irid, a whole bunch of them were marble, some turned out white/pale. Some even said it had dragon background . . . . but no dragons appeared. So I'm guessing everything I've read/heard about them are false.
But the platinum is a steel blue background with white opaque (this is also a steel blue background) and metallic.
SB x platinum : I'm guessing you will get black with irid, white opaque, maybe platinum . . . in the least.
OP: isolation is the best way to get them aggressive.
But sometimes they need time to realize the other color is also a betta. Usually whites have light colored siblings and some may not immediately recognize dark colors as bettas. Perhaps, letting them be together for a few minutes at a time may help (the male must be aggressive, otherwise a challenge from an unknown color may stress him). Once each recognizes the other and both are aggressive enough, then use the usual method (your usual method) of isolating - introducing - breeding.