We have to remember that even though the fictonal station 51 is in a urban area, and in real life probably wouldn't be called to a brush fire...at least not in the first alarm, but since wer'e talking about Emergency their district consists of the beaches to the mountains so they might be called in...like they were during the show.
As to the units sent...I believe a first alarm would consist of several engines, a medic squad, two or three camp crews (people that respond on brush fires, either paid or inmate crews...from the jails), a battalian chief...possibly two of them and one or two helicopters.
Then as the first in units get to the fire the amount of equipment will be either reduced (cancelled) or more equipment added as needed.