The REF won't generally attack cities located in the interior or on the side of your continent farthest from Europe. And, if your constitution frees the slaves, you get two indentured servants (ISs) from the freed slaves upon declaring independence. So, before declaring independence, colonize this "safe" area with tiny cities of population 1, each worked by a converted native (or a junk unit if you run out of CNs.) Park wagons loaded with guns (and to a lesser extent horses, if you colonized the city with a scout) in these cities. When you declare independence and get your two bonus citizens, run the wagons around from city to city, immediately turning the ISs into dragoons and rushing them off to the coastal cities that need to be defended, then scoop up all the remaining horses and guns in the wagon and go to the next city with ISs to be equipped.
Thus, each CN (who can't be used as a military unit) can generate two combat units. Plus, with the Betsy Ross FF, you'll generate enough cloth and saleable goods from the city square plus the square being worked that (depending on the tax rate and productivity of the squares worked) every 10 or so of these cities will generate enough cash from trading with Europe to allow you to buy a reinforcement from Europe per turn.
Obviously, you'll want to have monarchy in your constitution and at least one port city that is usually unblockaded to maximize this strategy.
Thus, each CN (who can't be used as a military unit) can generate two combat units. Plus, with the Betsy Ross FF, you'll generate enough cloth and saleable goods from the city square plus the square being worked that (depending on the tax rate and productivity of the squares worked) every 10 or so of these cities will generate enough cash from trading with Europe to allow you to buy a reinforcement from Europe per turn.
Obviously, you'll want to have monarchy in your constitution and at least one port city that is usually unblockaded to maximize this strategy.