The only rule: make them readable and obvious so nobody has to ask wait, what is that again? every turn.
While it might be situational, don't be surprised to see some trainers try to find a way to make Umbreon ex work within the competitive Pokmon TCG scene this year
The deck looked incredible, so many of the key components Azusa, Primeval Titan, Hive Mind, Amulet of Vigor, and more began to spike
There's a gas giant being studied by the jellyfish-like Illvoi, and a once-icebound world being terraformed by the bug-like Eumidians
TJs recommended decklist: DECK #2: ALAKAZAM One of my personal favorite Pokmon, Alakazam is a really interesting, mostly single-prize deck that draws a LOT of cards thanks to the abilities of the entire evolution line, and the ability to use the excessive card draw to swing for big numbers
Don't worry too much about why you're picking those cards out or if they will work together at all