Learn English – “hundreds of years” or “hundred years”

Yes, people normally say

fifteen hundred instead of one thousand five hundred

and

nineteen ninety nine instead of one thousand nine hundred ninety nine

However, I hear a lot of people saying

two thousand fourteen instead of twenty fourteen

Probably because twenty fourteen just doesn't flow off the tongue as nicely as years back in the nineteen nineties

"I have two twins/a set(pair) of twins" isn't ungrammatical, but it's more common and idiomatic to say "I have twins".

As for two sets of twins or two pairs of twins, both are correct. The former is a little more common.