Some comments:
1. The whole point of the Frontiersman is that he not only scouted new territory, he Settled it. I would be tempted to make this unit a Settler with a defense factor, so that it can be pushed into unsecured territory to found a new city/settlement.
2. "Minuteman" would be a purely Militia Unit, raised, perhaps from the civilian population but with lousy factors. On the other hand, from the late 18th century on, Americans and the US Army raised Riflemen as special light infantry troops: Morgan's and Berdan's the 2 best-known in the Revolution and Civil War, respectively.
3. The Constitution and her sister ships were referred to as "Heavy Frigates" and I believe when the Royal Navy copied them they also used that term.
4. Gatling Gun doesn't unlock all that early: patented in 1862, first service model 1866, and Hiram Maxim's 'real' recoil-operated machine gun patented in 1883: about 20 years in an Era in which the turns are usually more than 1 year (I think they are 5 - 10 years, but don't quote me: I haven't even opened the game in months). That gives the American player about 1 - 3 turns of 'early' machinegun, and the hand-cranked gatling was about the most unreliable weapon ever used: it was virtually guaranteed to jam within a few minutes of firing Every Time.
5. The USS Monitor and all her sister ships (about 60 total) were all coastal types and a fair number of them sank trying to prove otherwise, including the Monitor herself. To compensate they'd have to have really good combat factors, which given their relatively heavy armor and heavy guns is not out of line.
6. The US Army Ordinance Rifle was one of the first explosive-shell-firing field guns, but neither it nor the Parrot siege guns were that much better than the Krupp cast steel rifled guns or Armstrong British rifled artillery being manufactured at the same time.
Mine own list is slightly different, having been a unit historian in the US Army a few decades back:
*Homesteader - an 'armed Settler' similar to what I suggested for your Frontiersman
*Mountain Man - the Unique Scout that can Trade with barbarians as well as fight them
*Yellow Leg - the American Cavalry of the mid-late 19th century, able to fight both mounted and dismounted, the most versatile mounted troops in any army of the period.
*The Old Breed - the early 20th century US Marines. Get a unit for every X combat ships, infantry that has better combat factors and the factors are not affected by losses.
*FDC (Fire Direction Centers) - Atomic Era Unique Upgrade for all US Artillery, allows each unit to fire twice in a turn or artillery units in adjacent tiles to combine their factors into a single Time on Target Concentration on a single enemy unit. Can you spell "evaporate"?
1. Should Frontiersman replaces regular settlers in this case?
2. Minutemen is heavily associated with New England States (Mass. and Maine), and George Washington (who once lead either British Army 'Redcoats' Line Infantry or 'American Provincials' (Blue Coats) of the Seven Years War) despises them as being no so discipine. he's more inclined in raising REAL military unit similiar to British Redcoats with the same tight discipline.
3. Did 'Heavy Frigates' becomes 5th Rate in British System if British Royal Navy did reverse engineer it?
The US Navy uses them as ad hoc Ships of the Line. are these actually retains the same speed as Frigate standard of that time and does it has the same firepower as any real Ships of the Line?
4. Also Gatlings are exported. R.J. Gatling even has his factory in Britain and I THINK international export comes from there. any army of 1870s (except those armes forces of the United States) tend to use ones from British Exports.
it doesn't exists as standalone battery though.
one of my favorte gatling gunmount is atop of War Elephants. however gatling men came from Royal Siamese Navy and not Army (and thus they wore Navy uniform).
Soldiers to the left are from Army. I don't know when did fallcollar uniforms like this replaced standing collars of the Napoleonic era. though no generic soldiers of Napoleonic eras wore fallcollar tunics/coats with Shakoes. what Civ6 redcoats are shown in game is sometimes with 'mismatched uniform'.
I think the military advisor might be American. this kind of tunics look very akin to American Civil War.
Men to the right came from Navy, they're 'Marines'.
Wait. did Gatlings a compulsory or optional regimental guns for the Late Victorian armies and navy? does it worth a tech upgrade for 'Rifleman' or 'Fusilier' unit if 'Tech Upgrade' rule is implemented.
5. What is the first oceangoing 'Ironclad' as 'Line of Battle Ship'?
If Ironclads should exists as two separate units. one that FXis likes to represent are brown water types (riverine and coastal, some even belongs to Army rather than Navy! how wierd Army tradition is applied when running NAVY unit

).
What should be then name (and shapes) of oceangoing Ironclads (which wihin half a decade evolves so fast, into 'Pre Dreadnought BB' thing and first used RML guns.)?
6. Also about pre-recoilspring rifled artillery. How does the likes of Armstrong 100 Pounder do against armored ships like that? do these guns really have a good AP factor over smoothbore columbiads (distance cousins to big bombards of the 14th Century) of the same weight ratings?
Does it worths a 'anti-naval' siege unit?
And what's your view about Krupp (or Kruppstahl using) big guns of 1870s including ones that Bismarc toppled Napoleon III? did they still classify as the same thing as Civ6 'Artillery' ? or 'Rifled fieldguns'?
7. Tell me about 'Field Cannon' uniforms shown ingame? Is it Napoleonic or Victorian era? I can't recall if this kind of tunic (no tail) exists in early 1800s
Is this also uniform for rifled cannon crews as well?