Just one minor suggestion - If you do want your fics to be read, don't use obscure series. Moreover, don't use obscure characters. A multi-crossover has a higher chance of success if the readers know all the different fandoms you've mixed in...and the way to achieve that is to use mainstream series only.
Put it this way, the probability of a reader knowing ALL the fandoms you've mixed in diminshes as you add more fandoms. And the more obscure the fandoms, the farther the probability drops.
There are two ways of going about this -
1) Use utterly mainstream titles only such as Naruto, Evangelion, Suzumiya Haruhi no Yuuutsu, Negima, etc. Mainstream titles have the largest number of fans and there is a larger chance that a fan of one title may be the fan of many other mainstream ones.
2) Specify a certain genre and use only popular series within that genre. Say, mecha - use NGE, gundam wing, gundam seed, and other popular mecha titles. This way, you'll be targeting mecha fans who would most likely have a broad knowledge of mecha titles. Say you decide to go with 'school life', you'd use School Rumble, Suzumiya Haruhi no Yuuutsu, School Days, Higurashi no Naku Koro Ni, Kanon, Clannad, etc.
...but if you're only writing this for fun and you don't mind if nobody reads your fics, then I suppose you could do whatever you like, really :lol: