In English, I would say:
1900s - nineteen hundreds
1910s - nineteen tens
1940s - nineteen forties (or "the forties," more casually)
1950s - nineteen fifties (or "the fifties, as above)
2000 - two thousand
2000s - two thousands
I know there has been discussion about how to refer to the 2000s. But I think I would continue to pronounce it similar to past decades. "Two thousands" does sound a little strange just because it's relatively new. (Not so new anymore, I guess! :) )
I am not totally sure what the other part of your question refers to.
Do you mean more ancient times? If so, I would just pronounce the BC and AD designation (birth of Christ) because that is what we use here typically.
700 BC = seven hundred BC
300 AD = three hundred AD
I'm not sure I've heard someone say simply "in the two hundreds," for example. Just "two hundred BC" or "two hundred AD," depending. However, I'm not very knowledgeable about this. So maybe someone else can address this issue.
(native speaker, USA)