I tend to believe that the Earth Starfleet had dozens, if not hundreds, of ships even by ENT's time. They just weren't all as fancy as the ones we saw. The bulk of Earth's spacefleet probably looked like this
As for the question of half ranks: Again, just because we rarely saw them doesn't mean they didn't exist. Did we ever see somebody get promoted straight from Lieutenant to Commander? Or Ensign to full Lieutenant? No? Then it's possible that the ranks of LTJG and Lt. Cmdr. do exist. It's not like the costuming department couldn't make the appropriate insignia if they felt like it. I mean, Mirror T'Pol was a LCDR, after all...
The impression we got was that Krim was a really big boss in the organization, although not necessarily the very top dog.
Krim was cool. He was the very model of a modern Bajor general.