Regarding the subject of the ralts line’s egg group, this ask should answer your question, anonymous: http://bills-pokedex.tumblr.com/post/150226460696/why-isnt-the-ralts-family-in-the-human-like-egg
That said, birth defects are generally rare, and given the fact that a breeder is loaning you mate for your gardevoir (breeders are usually very careful about what they use as breeding stock), unless there are genetic defects with your own gardevoir, it’s unlikely that you’ll have to worry about that at all. Still, be sure to keep the egg warm and in a well-ventilated place to ensure that the hatchling will be healthy.
For those curious, in the meantime, ralts are extremely fragile pokémon, so it is true that defects are possible and that many of the possibilities have to do with this species’ fragility. Premature hatchlings are often too frail to live without vigorous assistance via artificial support at first, often due to immune systems that haven’t fully developed yet. However, that’s the generous side of things. There are plenty of defects that render a hatchling unviable for life at all, usually involving malformed organs or organs that haven’t fully developed at all. In rare instances—particularly crossbreeds with members of the gastly or misdreavus line—that organ may even include skin. Additionally, because of how small and slender a ralts body is, you may see a ralts with its internal organs squished one way or the other, but again, this is incredibly rare and only really occurs when both of the parents carry certain genetic anomalies.
But once more, all defects within the ralts line tend to be rare, especially if you’re working with a breeder’s pokémon, so it’s very likely you’ll simply have a happy, healthy baby ralts. (It should also be noted that the diversity or lack thereof of the egg group doesn’t have anything to do with the frequency of birth defects among hatchlings. If that were true, then we humans would be in quite a bit of trouble, given the fact that we have only ever been able to mate with one species, right? So long as pokémon are not genetically related—and given the fact that many members of the amorphous egg group are quite common, it’s unlikely you’ll be forced to breed blood relatives—the hatchlings will typically be fine.)
That having been said, best of luck and congratulations, anonymous!