Another note on evolution. You can evolve monsters literally as much as you want to. Any monster, including a 1-star monster, can be evolved until it eventually becomes a six-star monster. This is the more time consuming way to build a team but it’s arguably a little bit more fun this way, because who doesn’t want some weak little Low Elemental or whatever to reach a ridiculous level of power.
There are quicker ways to make the evolution happen as well. Angelmon and Devilmon, when they are acquired, are often already at the maximum level. Save all of your Angelmon and Devilmon. Go spam farming in the Cairos Dungeons or anywhere else that you tend to find a lot of Angelmon and Devilmon drops to rack up a big load of them.
The reason why they are so good for evolutions is this: Once you acquire them, they are already at the maximum level, so they can then be evolved together immediately. You can use them as evolution fodder for each other or for any other monster that you need to evolve, which will save a massive amount of time.
You can evolve any monster with any star color. Silver star monsters can be evolved all you want, up until a six-star rating, they just can’t be awakened at any point in the game. Gold star monsters can be evolved all you want as well. Awakened monsters can be evolved and will still keep their awakened form. So you can have a purple-six after starting with a gold one-star, theoretically!
When you’re really having trouble finding some rare ones, spam farm the boss battles over and over again to grind for 3-star monsters. This is another recommended strategy for when you have an evolution coming up.