I believe the AuxPoW fork code allows the sharing of any script coin with a hashrate higher than that of Doge, which at this time is only LTC.
I don't think that's correct at all. Any scrypt coin has to have the merge mine patch added if they wanna be merge mined with LTC and doge. There's nothing automatic about it.
I'm not a coder but I know That's how it's done with BTC and all the merge mined coins there.