"You haul for hire, you need a CDL and even a DOT number." I'll address this as interstate transportation (intrastate transportation is governed by each state). This statement is not entirely correct. You only need a CDL if you are in the furtherance of a commercial enterprise (private or for hire) and the GVWR, GCWR, or actual weight of the vehicle (or combination of vehicles) you are operating exceeds 26,000 pounds, or you engage in certain passenger transportation or certain haz-mat transportation. The registration requirements for USDOT numbers begin at 10,001 pounds.
Example: If you have a small box truck with a GVWR of 12,500 pounds and used in business (for hire or private) you will need a USDOT, DOT physical, and be subject to the Federal safety regulations. However you will not need a CDL (except perhaps Haz-Mat).
One thing that adds to the confusion is that the definition of "commercial motor vehicle" in CFR 49 part 383 that deals with driver's licenses is different that the definition "commercial motor vehicle" in CFR 49 part 390 that covers Federal motor carrier regulations.
For reference I am a retired DOT inspector who sat on the vehicle committee of the Commercial Motor Vehicle Safety Alliance.