[quote=@Raddum]
Perhaps?

There's a fair amount of crossovers and references between a lot of these shows that could provide existing relationships in one way or another by default. (KND and Grim Adventures being one of the older ones I can think of; Specifically the one where Grim loses his powers and Numbuh 3 takes his job for a 1 month trial period.)

[url=http://cnmultiverse.wikia.com/wiki/Crossover_Episodes]This list[/url] Coverssome of the Cartoon Network crossover episodes over the years. Could be a good place to start. 
[/quote]



Cartoon Network Invaded, I totally forgot about that. Though, I question the validity of some of those entries.

But, yeah, this is mostly helpful.