Okay im not sure whether it got answered already
Yes, it did. In general, the answer is ALWAYS yes on that.
is it possible to have more than 2 unique units for a civilization?
Yes. However, the introduction leader screen (and the Setup Game screen before it) seems to be limited to two unique units/buildings per civilization, so it won't display them all. But you'll get them, from what others have tried.
After all, this is essential to the various fantasy mods; the Elves won't have the same units as the Dwarves and such. You could take this to the extreme, where ALL units are unique to one civilization, and it'd still work as long as they're balanced well. (Starcraft mod, anyone?)
Alternately, consider the Age of Empires approach, where each unit or building is used by half the civs and not the other half; it's the combination of all of these that becomes unique. This'd take a LOT of bookkeeping to assign the units correctly, but it can be done.
Or imagine a mod where certain buildings are split by "region", like with some civs getting a Temple, others a Shrine, others a Church, and others a Mosque, each with slightly different (but balanced) stats.
All of these are possible to mod in, if you're willing to do enough work. It's just that one UI screen that can't handle it.
And also is it possible to have more than 1 leader for a civilization?
Yes. Effectively you just create "AMERICA_LINCOLN", "AMERICA_OBAMA", etc., all with the same unique ability and unique units, same Civilopedia entry, same background music, and so on. It wouldn't take many changes.
In fact, you could easily tie this to the earlier question. Maybe Washington's America has the Minuteman and B-17 as units, while Lincoln's America gets the Conestoga (improved Settler) and Marine (improved Paratroop), and Obama's America gets the F-22 and Wall Street (yes, you can make a unique wonder).
The downside is that the game wouldn't see these as being related civilizations; it'd treat them as unique, so it'd be possible for a game to randomly select all of your America variants at the same time. Besides just being a bit disturbing in general (Lincoln and Washington firing nukes at each other?), I'm not sure how the game would handle them both using the same city names, colors, etc. (Maybe the second gets a random color?)