A staff weapon could indeed be built fairly easily. Shooting BBs out of it won't be nearly as cool as shooting "kill everything" energy blasts, but still pretty sweet. Only problem is, you'd have a hard time outfitting an entire team with these to get an authentic Goa'uld force.
Not sure about making a working Zat, the profile of it doesn't really have much room for a gearbox or a gas system. I could be wrong, but it seems like a tough one.
As for SGC weapons, here's the rundown as I remember it.
SG teams generally use MP5s (or variants of them) for exploration/recon missions, with some exceptions. Combat and Marine teams use M16s or M4s, in pretty much any variant required by the mission. SG-1, being special, use the P90, because the producers thought it looked "sci-fi and cool". Atlantis teams usually use the P90, or the G36 if they need more firepower. For sidearms, most SG personnel carry an M9, but 1911s are also used. There are a lot of different weapons, really, anything the Air Force uses is fine (and even more).
Oh man, I wrote that whole thing and then found this. Has a list of every gun seen in the series.
http://stargate.wikia.com/wiki/Firearm
As for SGC uniforms, SG teams usually wear flat color BDUs and black tactical vests. But there are lots of exceptions, including the Marines, guards, etc.
I'm trying to think of games for this in which people won't have to custom create their own guns, because that would seriously reduce turnout, you know? That's why my first idea was earth-based (Sure it's not as exciting as SGC vs. Goa'uld, but really, where are you going to find an entire team of Airsoft Jaffa?