The same argument can be said about Steel and Dark types being discovered in Gen II(which is after Gen III according to some of you), yet we have both of those types available in Gen III quite normally.
Btw, I'm not saying they have to be inbetween the Unova games, I'm saying that there are also things that go against it happening simultaneously with Gen I.
The fact that Dark and Steel types were introduced in Gen 2 has nothing to do with the timeline. Since Magnemite is a Electric/Steel type in FR/LG and is a mono Electric in the originals. It's just a new mechaninc.
Well I believe that R/B/Y/FR/LG happened first and that G/S/C/HG/SS and D/P/P happened 3 years after, B/W happens after those games since Cynthia mentions Lucas in B/W and B2/W2 happens 2 years after B/W.
R/S/E could happen arround R/B/Y/FR/LG or it could happen around B/W. I think they happen around R/B/Y/FR/LG.
Also Gamefreak could pull a Zelda Timeline on us...