Seeing as differences in lore and whatnot may cause problems, we may as well pre-empt them. I suggest the adoption of the following decalogue to handle these events when they occur and (hopefully) keep them civil, structured and (relatively) organized.
1: The GM of each game and the Creator of each setting has absolute control of it. They have the right to dictate anything regarding entities from their domain. In the absence of a GM or Creator, an official rulebook or wikia will suffice, the players who use material from that world have the right to interpret the lore as they see fit as long as the absolute majority of other such players agree to it.
Case in point, @Elf has absolute control of RotMG and @Ragnarok has absolute control over TA as their respective GMs. @Mcclay has absolute control of Eric's world as its Creator and @Vgray has unquestionable right of interpretation of Warcraft as the only user of that world.
2: Unless an agreement is reached, the worlds of each setting are to be considered completely separate with neither influencing the other. An amalgamation requires the consent of both GMs or Creators. In the absence of either, this requires an absolute majority of players with characters from those worlds to agree to it.
Impolitely jamming lore into someone else's lore probably isn't the best idea, especially if they are incompatible.
3: If a lore difference occurs which nullifies a character's ability, the GM or Creator of the relevant world is to provide an equivalent effect, either lore or mechanics-wise if possible.
For example, if someone who could detect magic signatures used it upon Florencio, they should receive the echo data of his recent spells as he has no such signature. Likewise, were they to use it upon Silica, they would receive the words she used in her spell.
4: When multiple entities are omnipotent within a domain, any such entity can veto the actions of the others, regardless of numerical advantage.
Infinity multiplied by one hundred is still infinity. Thus, no matter what a group of entities with infinite power does, a single other entity could simply undo it as numerical differences become meaningless. By that logic, when two contradicting actions are backed by omnipotence, the status quo wins.
5: There are no grandfather clauses.
A person who could veto an action may miss it or not be online at the time, this is by no means an invitation to assume that it is a precedent or permission to utilize it in the absence of an explicit statement.
6: The GM or Creator of a world has no obligation to provide any lore when asked.
It can be used to explain or justify something, but if for whatever reason they refuse to, whether because they haven't completed that lore or because it would be a spoiler, that decision is to be respected.
7: Should the lore of two worlds end up in direct conflict, the relevant parties are to either reconcile it or nullify the action and any required parts of the sequence of events.
Unbreakable objects going against forces that destroy everything can't exactly be resolved easily, so failing resolution, the event in which they interact in a contradictory manner is to be removed to prevent the contradiction from occurring.
8: All absolute property arguments are to automatically be relative to their world. Properties that destroy or erase are automatically defeated by properties that allow survival.
That is, something that destroys everything without fail only does so relative to their originating reality.
Having an indestructible object suddenly become destroyable tends to compromise settings far more than an all destroying effect failing. After all, it is far easier to invoke and abuse (and possibly kill characters using) the former rather than the latter.
9: Unless otherwise stated, relevant aspects of each world is to be assumed to exist.
That is, there is void in the world, there is a television world, there is a border of life and there is a spell server.
10: The crossover itself is not subject to these laws.
There is no point in to futilely questioning how characters from strictly separate realities ended up in the same location nor whether or not it violates their lore.