Hello there, the search function should reveal a few of your answers, but it'll take some work. Suzuki's are renouned for their interchangable parts, most of them will either bolt straight up with no or very little work. The 250 and 400 weigh just about the same, and the only major difference being the engine, carbs and airbox. I don't see why the rear shock would be changed between the two, and they both look very similar. Are you looking at that as an upgrade? If so I wouldn't bother. Have a look into gsxr shocks. There is a sticky thread on here somewhere that lists suitable upgrades
If you need a 400 shock measuring I can do so, as I've a dead old one lying on the side. I've replaced it with a Hagon Monoshock. Bit costly, but well worth it.
As for the plastics, it would seem the seat fairings are the same (both gens), but the airbox covers on the Mk1's is different between the 250's and 400's. The Mk2 airbox covers may be interchangable, however they could have some bungs in slightly different places, due to the airbox shape. I would also believe that the tank taps are the same for the 250/400's. I am in no way guarenting any of this, but it's most likely. Another way you could check is to look up the item codes for each part. If they're the same, then suzuki didn't change that part, and they are interchangable.
Sorry I can't be of much use, but the 400's are not well documented, let alone the 250's. What year is your 250? That should aid your search.
All the best,
Bullet5.