Absolutely, Zat's have replaced the Standard Issue .45 caliber sidearms for all SG teams.
One of my favorite lines is when,after Carter demostrates the superior accuracy of a P-90 versus a Staff Weapon against a target. She cuts the log in half under full-auto, then switches to single fire and proceeds to cut the still moving rope with a single shot, Jack holds a staff weapon up and says,"This is a weapon of terror. It is made to subdue through fear.", then holding up his P-90,"This is a weapon of war, it is made to kill people" I was giggling uncontrolably for several minutes.
As far as everyone speaking english, it's probably a widespread language among the Goa'uld placed human colonies as it's relatively easily learned and can transfer a larger amount of information per word. Or they could have Universal Translators, one of the biggest gripes about Star Trek is how everyone speaks english. They don't, they use UTs. ST just removes the step of the UTs providing a translation.