With some exceptions, most of your rolls will be random mons generated from 1 - 3 species of Digimon, Pokemon, and/or Yokai possessing up to 5 types and an attribute. That can be a lot!
So make sure to focus on the fun of the fusions, and remember, just like with pokemon, not every element of a monster will be equally obvious. Not every electric type needs to be yellow and not every water type needs to be blue.
That being said, do keep in mind general themes of each type, and do your best to incorporate something from every element of your creature.

Included in these guides is a non-exhaustive list of common themes for each type and attribute, to get you started, but in my mind, so long as you can make a meaningful connection between the elements of your monster, you're good to go!
Furthermore, while not something you may want to use often as it is a cop-out in some sense, sometimes the personality, narrative, etc of a monster may be enough to connect it to its type. Fire type mon that doesn't have any red or fire but looks pissed off 100% of the time and picks fights with anything and everything? Yep that's a fire type.

Unless something truly egregious is submitted I will not be policing your monster designs.

It's simple, I don't have any interest in infringing on fun and creativity, and at the end of the day, that's what we're here for.

If you want help, I will of course give you pointers on how I feel you could push the design towards a certain aspect, but if you want to do a bright yellow dark type, I'm not going to stop you.

A Special Note on Legendaries and Mythicals

Legendaries and Mythicals are in the game, and are treated as special but not singularly unique. There is more than one Ho-oh, more than one Suicune, etc.

There will be no specific uniqueness to legendaries beyond how hard they are to get, especially as Digimon and Yokai each use entirely unique systems for their "legendary"
There is one exception to this, and that is the digimon "Calumon" who will be awarded per trainer for completing a specific prompt and cannot be bred or traded, unless someone makes me an insnanely good argument for a trainer posessing more than one, each trainer can have a max of 1 Calumon.