Yes, this EMP campaign will include firearms; it's the 17th century after all. They will be available for PCs and NPCs alike, with no vague prohibitions. Anyone who can afford one, can be trained in its use, and take care of a firearm can employ them as they see fit. I don't see any problem with having this piece of technology (or any time appropriate technology) enter into a fantasy game.
What follows is descriptions of firearms as they were known in 1618, consolidated so as to be usable for gaming without the minutiae. We'll get into mechanics in a follow up post.
Firearms are broken down into a number of categories:
Barrel type - smooth bore versus rifled
Firing mechanism - match lock or snaphance
Size - primarily to differentiate between infantry and cavalry weapon
By far the most common firearm by 1618, muskets are smooth-bored and muzzle loaded. Smooth bored firearms are easier to load and are much less expensive than rifled barrels at the cost of accuracy and range. Muskets are used heavily in warfare for 1618 but can also serve an adventurer.
While caliber sizes were not standardized at the time, we will assume two different calibers, .59 and .80. The .59 caliber musket, the caliver, could be fired without the aid of a rest, as opposed to the heavier .80 caliber musket.
The rifled barrel is an improvement over the smooth bore in terms of accuracy and range. A tight fit between barrel and shot is required to achieve the spin that increases accuracy and range; as a result, rifles have a significantly lower rate of fire than muskets. Originally used by hunters who only need one shot to make a kill, they are slowly being incorporated into armies, particularly for sharpshooters, and have found a niche among some adventurers.
The Hunter Rifle comes as a .65 caliber firearm with one or two barrels.
The economical choice in firing mechanisms, the match lock uses a slow burning match to ignite the powder in the flash pan. Obvious drawbacks include the need for a lit match and an exposed flash pan which could both be problematic to say the least. However, they are a common firing mechanism despite this fact due to their ease of manufacturing and low cost, meaning they saw wide use in many armies.
The snaphance is the latest firing mechanism to utilize flint and steel. The complex mechanism utilizes a spring loaded flint that strikes steel while simultaneously uncovering the flash pan. By totally eliminating the need for a lit match, a snaphance is extremely reliable compared to match locks. Owing to its complex nature, a snaphance is relatively uncommon in military usage but more common for adventurers who can afford them.
Long barrel firearms are mostly unused by cavalry; single handed firearms are preferred so that a rider can still hold his reins. While they can be aimed and fired with one hand, they are still difficult to load on horseback, meaning most cavalrymen took at least two pistols into a battle.
The standard military pistol is a .55 caliber weapon with an 11 inch barrel.
That covers the types of firearms that will be available in the EMP campaign. Firearms will be "built" by choosing a barrel type and firing mechanism. The more 'advanced' firearm technology will slowly become available as game years progress, bringing the flint lock and paper cartridges into usage. While breech loaded weapons were in existence by the early 17th century, they are essentially unavailable at this time.