I'm actually going to be doing a Secret of Zir'An game tomorrow.
I've also run a d20 game for the past year that's had magitech firearms in it.
Can guns work in a d20 (or other fantasy) game? Sure. Note I said "guns", not just "firearms". It's something to think about. The thing I've noticed anytime gunpowder seems to come up in the fantasy context is a lot of contention about how it'd completely change the world or just wouldn't be possible because "realistically" blah blah blah.
Stuff that noise.
Do you want guns? Then decide how they function. In my personal opinion you can certainly have pure "magitech" guns which are basically firing energy bolts. Or you can have "firearms" which are relying on a little bit of science (or pseudo-science) in order to kind of "start" things, and then magic which directs and controls it.
Or you can have something like gunpowder having been inadvertantly created by an alchemist. Instead of developing a lot of other supporting science, metallurgy, and all that other stuff, they simply use magic to hold it all together. So basically they pour a bunch of powder into the weapon, and set it off. Ordinarily it'd just explode, but thanks to the runes and other bits, it instead propels a projectile.
Messing around with stuff like gunpowder/firearms can lead down the rather nasty road of "realism". I say "nasty" because at the end of the day, you're playing a game and it's set in a fantasy world. Which means it's not going to be realistic. As you strive for realism, it starts to spread it's tendrils and start going into all kinds of crazy directions.
My game, it had the magitech energy bolts, as well as "thunder guns". The thunder gun isn't actually using gunpowder though. Instead, the projectile has runes carved upon it. When energy is spent, the projectile (it looks like a musket ball) leaves the barrel so rapidly, there's a void of air left behind which creates the thundering sound that the weapon is named for. The weapon itself has 6 barrels which are manually rotated in order to bring the proper barrel into alignment for the energy conduit to launch the projectile.
Your biggest problem is likely going to be damage. How do you handle it? You're going to have to make a choice, and that choice is going to bother at least _some_ people.
If you give it equivalent damage to a bow/sword, you're going to annoy the group that feels the weapons should be so superior to the "archaic" weapons.
If you give it better damage, you're just causing people to ask "Why would anyone ever use another weapon unless they were forced to?" You're also going to have to potentially deal with a cascade of armor issues and other bits.
The game I ran had the weapons operating on the same level as swords and bows. Essentially it was more of a "special effect" than a matter of superior weaponry. Of course, clever players will be able to use special effects to their advantage as well. I wouldn't sweat it personally, and in fact I didn't sweat it.
The choice between the magitech and bows was a bit of a tactical one for the players. The weapons did have their own magic source, but it has to be recharged _from_ someone. So if they're recharging their weapon, it meant they weren't able to cast spells or use other abilities that also used energy.
Bows on the other hand, you just buy more arrows. No muss, no fuss.
Thunder guns have the issue of the ammo. You either have to carry a bunch of replacement shot, or you have to try and track down the used projectile and hope it's in a condition to be re-used (just like with arrows/bolts).
If you're looking for some ideas, you should try and pick up "Arsenal" from Perpetrated Press. It's pretty easy to find it online, and cheap too. They've got some good ideas for magitech weapons and how to handle guns in a fantasy setting.
Oh, and good choice on picking up Secret of Zir'An.
It's not without its share of quirks, but no game is perfect.
