CA, like in all things, is the biggest pain, but all of the details mentioned above are nationwide DOT rules. The states sign on and enforce the same basic rules but CA takes it farther because the state government is run by socialist do-gooders. All the DOT rules apply in OR also and OR has strict enforcement.
The 5500 and trailer combo will put you into Class A CDL territory and all rules apply. Log book, 70hr/11hr driving limits, driver's physical and medical card in your possession, DOT numbers and company name displayed on both sides of the truck, triangle reflectors carried, fire extinguisher installed (not carried loose), commercial vehicle inspection decal on the windshield, vehicle registration and $1M single limit liability insurance card documents handy and ready to present, everything properly chained or strapped down, mud flaps, safety chains crossed under the trailer tongue, safety breakaway lanyard attached to a permanent mounting point, not draped over the tow ball, vehicle inspection sticker on the trailer, proper DOT lighting on both vehicles, no lamps out, good tires, good trailer brakes with brake controller, clean windshield, good wiper blades, no windshield cracks in the driver's line of vision, no dark window tint film applied, and more. Driver cannot sleep in the backseat, must stop on all scales, etc. I've been off the road over a year and have forgotten some of the rules already.
Best advice I can offer is pay a visit to your local DMV office and obtain a commercial driver's handbook, read it carefully with your boss, discuss and get questions answered by a DOT officer, and COMPLY.
As FFeiller wrote above, all the internet forums are populated by guys with diesel pickups who are self-proclaimed professional truckers who think they are experts, practice none of the rules, and think they know all the ingenious ways to skirt the rules and get away with it. They often do get away with it for a trip or two or if they stay on the backroads and try to remain invisible but I've talked to a few of them in truck stops who had the stack of tickets to prove they were wrong and it was funny to listen to them whine. A DOT officer will chase you down if you run a scale. If a DOT officer catches you with the 5500 rig not in compliance he'll write you a big stack of tickets and put you "out of service" for at least 10 hours. After 10 hours has passed, depending on cited violations he may allow you to drive away with the truck but the trailer will sit there until a Class A CDL driver with all the requirements met shows up to take the trailer back to OR. Fines can be severe.
The distinction is 26,000 lbs. gross combined weight or gross combined weight RATING. If you use the pickup and trailer combo and the total gross weights or gross weight ratings do not exceed 26,000 lbs. you won't require a Class A CDL in other states but all the above rules still apply. In CA any flat bed or truck larger than a pickup must stop at all scales. A DOT officer will add the total of the truck's GVWR limit from the driver's door post decal AND the GVWR tag on the trailer or can scale weigh the vehicles, whichever is greater.