So i was thinking about Derek. When derek & addison were interns with richard, were they in seattle?

Mmh, bạn do have a point. But as Derek says that he hadn't realised that Seattle had ferryboats, and Meredith points out that Seattle is surrounded bởi water on three sides in the elevator scene in 1x02, I'd say Derek hasn't ever been in Seattle before.
There are 2 options:
a) the writers made a mistake.
b) Richard transfered to New York (e.g. since Derek and Addison had a private pratice there [not 100% sure though]) and meanwhile he was the resident of Addy and Derek.
I can't give bạn a 100% answer though. Sorry.