I've heard that Pokemon from different language games will be unable to pass on natures. If they're both English though... then I'm not sure. You're sure that the Ditto is holding an Everstone, right? That's incredibly important. If you've fulfilled all those conditions, then your only problem is that you're unlucky. 25 eggs? Really, the chances of NOT getting a Modest Yanma are like... 1 in a million. Seriously. Nobody's that unlucky, so I'm guessing you either are using a Japanese Yanma, or forgot to give the Ditto an Everstone....
Yup, that's exactly what I'm saying. Kudos to you for understanding it on the first try, haha, a lot of people get confused by it. But it's really an effective strategy once you get used to it, and slims down on the number of eggs you have to hatch/fill up your boxes with in order to get the desired poke.
By the way, when I say that "all but IVs are determined with the egg is produced," I mean EVERYTHING. This includes nature. So if you get a Modest Compoundeyes Yanma, scrap it since even if you soft-reset a million times, it will always be Compoundeyes. In this light, you have a 1/4 chance each egg to get a Modest Speed Boost Yanma. 1/2 to get Modest (passed down from Ditto), and 1/2 to get Speed Boost. Not too good, but a lot higher than if you were to go about breeding just randomly.
Unfortunately, I'd say that it's automatically useless due to the Calm nature, even if its Special Attack IV is flawless.
Do you soft-reset for your eggs? What you should do is, when the daycare man has an egg, save in front of him. Take the egg and hatch it. DO NOT take any more eggs from him. If the hatched egg is Modest but doesn't have Mischievous as its characteristic, soft-reset, take the egg again, and hatch it again. Since all but IVs are determined when the egg is produced (IVs are determined when you take the egg), once you get a Modest Yanma, you can soft-reset until you get good IVs.
If the hatched egg isn't Modest, then release it and save in front of the daycare man again, and repeat the whole process. The reason why you shouldn't take any more eggs after you take the first one is twofold: 1) you might not have another egg to take and soft-reset on if the first egg you got wasn't Modest, and 2) it has been proven that the more eggs you have in your party at once, the longer it takes to hatch all of them.
I hope that made sense, it's very complicated but this method of soft-resetting for IVs is a great way to get good IVs with the right nature quickly, rather than taking and hatching a whole bunch of eggs and hoping for the best.
Nah, I wouldn't doubt the legitimacy of a level 16 shiny Starly. A lv. 100 shiny Staraptor caught in Route 203, maybe. xP It's also obviously legit because of the same ingame name, I.D. no., etc as your Slugma. Thanks a lot! I've always wanted a shiny Starly. I hope you make good use out of that Ditto.
Do you have Wi-Fi? If so, I could trade you a Modest Ditto that has a flawless Sp. Atk IV (the other IVs aren't so good though). I caught it in FireRed so I can easily get another clone for myself using Emerald.