[@Killfang96]

Some people already explained it, but Guardians are custom 1-of monster cards that are basically this:

-Monsters you can summon but are NOT in either the Main or Extra Deck.
-Monsters of varying strength that can and MUST be summoned when your life points are 1/4 or below their starting value and certain conditions are met. (For higher level monsters (I'm thinking 7 and above), additional costs must be paid.) For example, a Level 7 monster might require 3 or more DARK Fiend-Type monsters to be on the field at the same time, and two of those monsters must be tributed for the Guardian to be summoned.

As for Custom Cards, you can have up to 3 to start off with (not counting your Guardian), and additional Customs can be given out for plot reasons.

In terms of what you can and can't do, as long as it isn't crazy powerful like Pot of Greed(which allows you to draw 2 cards for free) or The Forceful Sentry(which allows you to look at your opponent's hand and shuffle 1 card from it into their deck), it should be fine, but I will be looking over what everyone creates just to be certain.

Banlists are in effect, with some to-be-determined personal adjustments made by me.

And yes, you are allowed to use cards outside your archetype. So generic decks and decks focused around 1 type of monster are perfectly fine.

[@Darkmoon Angel]

Anime/manga cards are allowed, within reason. If it's something crazy that would split the game in half, like Number ic1000: Numerronius Numerronia, I'm not letting that slide under any circumstances. If it's a card that has been released in real life, the real life effect will be used.

So yeah, stuff like Fortune Fairies, Alchemy Beasts, and Scab Scarknight can be played.

Also, [@Killfang96], don't make rule assumptions before I say for certain, please.

[@AdmrlStalfos19]

With some Guardian-related tweaks, yes! Spirit of the Pharaoh is a pretty atrocious monster normally, so by all means, go for it.

[@JamesMuddy]

That's a pretty nice example, albeit without the 1/4 starting Life Points or less restriction or having the Summon be mandatory, but still a good example.

And 1 more thing that I brought up earlier in this post regarding the topic:

Guardians can and must be summoned as long as you meet the requirements to summon them, and don't necessarily need to have effects that Summon them when an action is performed that will cost you the game. Think of them as more of a Destiny Draw-esque mechanic from Duelists of the Roses and the Tag Force games, a trump card that will help improve a bad situation.