Back to Keiichi/Skuld, well, I won't be ranting about Skuld being too young since...well, to be honest, I think her age is...decent enough for a Keiichi pairing. Now in REAL life, I can see major problems (I'm assuming that her real age is the same as say, what she would be as a mortal. Around 13-15 perhaps) but, when I can find Evangelion lemons acceptable, I see no problem with Keiichi/Skuld. Anything younger, then I would have a problem.
As for Keiichi/Skuld, I have seen a fic at FFN that sort of had this pairing. It was a lemon though. And since you know what happened to NC-17 (now MA) rated fics on FFN, well, it's gone now and haven't been able to find it (bad enough trying to find places that host AMG fics. Finding MA fics are another matter).