Can I end up having 3rd or 4th rates with big guns rather than a dozen or so little ones? Or are you stuck with 'research the ship it will be loaded with a pre-set number of guns of the caliber of its class' type of shipbuilding?
Not sure what you mean by "big guns rather than a dozen or so little ones" but the way research works is that you research a certain technology that will give you access to a ship - a Drydock for a 2nd Rate, for example. To the best of my knowledge, all ships have a pre-set, more-or-less historical number of guns of the appropriate weight.
Also, are there any non-ship of the line vessels that can be built and commanded? Aka sloops, raiders, merchants?
Yes. Sloops, Brigs, various merchantmen such as Galleons, Indiamen and Fleuts, all the way down to galleys containing a single gun. There are also "modified" ships which can be captured from pirates. For example, in my current game, one of my fleets contains a "race-built galleon" which is essentially a stripped out, 60-gun, makeshift warship.
Merchantmen play what is perhaps the largest role in the game since they can be such a significant source of trade income.
Being a fan of the old tv series SHARPE's RIFLES I would love to know if there are any riflemen units in the game that can be built .. and if so, how differently do they fight vs line infantry?
Loved that show.

Yes, there are multiple rifle units. However, Rifling is actually a technology which must be researched. In both the early and late portions of the game, however, you have light infantry units.
These units are half the size of standard infantry units (60 men v. 120) but will typically have greater range and accuracy than their line infantry cousins.
They can be fought in two different ways - as a secondary unit of line infantry or, more importantly, once youve researched "Light Infantry Doctrine" - as a very effective 'fire and maneuver' sort of unit. In the latter capacity, the men will spread out in a random fashion and behave as skirmishers.
I actually find light infantry, once the appropriate technology is researched and tactics are altered, to be very effective ambushers and, as stated, they can not be beaten for their mix of high mobility, flexibility and constant upkeep of fire.
As a caveat, a modder has already released a "Sharpe's Rifles" English unit for the game. In game, you are permitted only the single unit of 60 men, who are dressed as they were in the show, complete with a fairly well integrated mug of Sean Bean in the unit card.
Aside from US and European powers, what other nations can you 'be' (i read you could be a native american nation... but are there others? south america? mexico? japan? australia/nz? ).
If memory serves, there are roughly 50 factions throughout the map. I believe 12 of them are playable at the start. Non-European states include the Ottoman Empire, Russia and the Martha Confederacy (India). Mods are available to unlock all factions, at this point, I should think.
Mexico is a good example of a history and game-dependent "emerging" state. Mexico is controlled by Spain at this moment in history. "Mexico," as a county, IS in the game, but for them to emerge, a rebellion and subsequent revolution against Spain ( or ruler at time ::twiddles thumbs:: ) must take place within the appropriate territory. Then the Mexican flag pops up over the city, guns go firing into the air and drunken mongrels mix it up with Clint Eastwood.
Revolution is a common theme in the game and is required to reveal many "modern" nations.
For its flaws - of which there are many - I love the game. I think, if you have a love of this period in history, you will also like the game. It satisfies a need of mine that hasnt been satisfied since I had a 386DX-16; it allows me to march a line of little virtual dudes against another line of little virtual dudes and mow them all down.
As a period-correct land battle simulator - its second to none, IMO. At $30, I dont see how you can go wrong. That's a couple beers and a pack of cigarettes, these days.