An assistant is an assistant chief, there is usually one in each battalion. In my setup I have them as battalion chiefs or MCV's.
OES is Office of Emergency Services which have units stationed all around California and help with incidents along with natural disasters.
ESU is a Emergency Support Unit which is a large medical support vehicle. In game I have this as an EMS support operations vehicle
Can be harder to tell, depends if it has a pump on it, can check videos and pictures. I usually go with a normal platform if it isn't a quint.