Any where you see dissimilar materials (cast iron to galvanized) you will probably find similar mineral deposits as you found in the clog at the sink piping, but do you need to switch everything out?? Hard to say really!
In my opinion, if you have time and you want to remove all the potential future headaches then it is probably a good idea to eliminate the old kitchen sink piping and any other 2" or smaller drains and replace with ABS if possible. Otherwise, in terms of the larger cast iron pipes, I wouldn't necessarily recommend replacing these unless there are obvious signs like cracks or moisture on the bottom of the pipes...cast lasts a longgg time if left alone! With that being said, however, if you have the time and want to replace all then there is certainly nothing wrong with that, too!
ABS piping curved? Curious...doesn't usually curve like that?? Where did you get the piping? If you want to try to straighten them I guess you could try leaving the pipes out in the sun for a couple days and see if they reshape, otherwise, I'd return them and get straighter lengths here...
Back to you..