well in Pokemon platinum you can just get the member card and get darkrai after getting darkrai you can get cresselia
A Dragonair and a Ditto will have a Dratini egg.
see the problem is that SUPPOSEDLY all ledgandarys are not a boy or girl and if they are then they arnt a ledgandary (according to my brother ) but also see the problem that cresselia is a girl and you can only catch one cresellia so the only way that you can breed it is with a ditto , and i am sure that the makers of Pokemon would not let you have a full team of cresselias or that would be " cheating " so if you where to breed a cresselia with a ditto then im also sure that the makers of Pokemon would probably make it takes weeks to make a cresselia egg so that you don't get a full team of cresselia's so quickly but they could have just said NO BREEDING LEDGANDARYS full stop so that you can never breed a ledgandary so the answer to your question is probably no send me a message if that was helpful ;) Comment: I managed to breed a Ditto with my Cresselia, and got an egg fairly quickly. The hatching took a long time, though, but it is possible.
No.
you cant make an egg with Garatina.
Yes.
yes
no, it can't
yes it can
yes ditto can breed with any Pokemon group except for those in the no egg group
yes,yes it can
sadly, no. legendaries cant breed.