Need more info...
If it is a newer 90 (c90) the electrical system is identical to the 1900. The 200 is a different system. the above post bout the cross start is not correct for a B200, just the older "straight" 200s.
If you want to get a head start, start with the pressurization stuff, powerplants (variations of the PT-6, -21,-42,-67) flight controls will be very similar, fuel systems (minor differences), Air Conditioning. Hydraulics will be entirely dependent upon which model of 90 and 200 you will be responsible for.
Another good word of advice: When learning any new airplane, the best way to stay ahead is to make some flash cards with the limitations and emergency procedures. Sit in the cockpit and touch the switches as you go through the memory items to familiarize yourself with the locations of the items and systems. That alone will keep you on track.
Hope it helps. I'd be happy to answer any specific questions you may have. I have very little experience with the older models, but I can help with the c90, b200, 300,350,1900...