if OP HAS a 20-24V (better be 24V) then by all means, 7818-7805 one feeding the other would be an option, he did mention small footprint requirement though.... and that would not be so with proper heatskinking... I used an 7805CV recently in one of my boards, powered from 7,5V and drawing 0,4-0,5A tops, it reached 100C within few minutes... no heatsink, simply laying on the pcb with the GND soldered to a huge pad on it...
if OP wants to build something that requires 18V and 5V like he described then i insist its a hell of a lot easier to find a proper 5V-2A wall psu on the cheap like the one i mentioned, and a small boost module would make sure his non-demanding 18V rail was met again, on the VERY cheap..... you cannot beat this logic i am sure LOL....
As for the remark about how laptop power bricks come at high voltage etc, you do understand the smaller laptop power brick starts at around 40W power right? And that most chargers over 15V are used to charge 4S battery packs so a voltage of 16.8+drop on voltage regulator and charger circuit is required... so it is only natural to design with a higher voltage in mind, when you really use most of the power in form of 3,3-5-12V rails (and not only)....