In Elvenbane, the point is made near the beginning that elves are not terribly fertile, feeling lucky if they manage to produce one child in a decade. This ties into the relevance of them being exceptionally fertile with humans, means that Dyran is exceptionally proud of himself for figuring out a magical way to ensure conception, and explains why the world isn't absolutely overflowing with elves (given that they live a very long time indeed). Yet the rest of the series, and even the rest of that book, doesn't seem to hold this up. Many elves with children have multiple children all born within a few years of each other, and there are piles of discontented non-heir young elves hanging around with nothing to do.
Not all of elves in the rebellion were second or third sons. Some were those who had little magic and hated the fact that the ones with more magical power treated them as only slighter better than humans.
How do we know they were only born a few years apart? There were probably a few decades between them at least, which would account for the multiple offspring of semi-powerful families. And too, only having two or three children in a lifetime is actually a pretty small number if you think about it, considering that elves, like humans, seem to be fertile almost year-round (as opposed to having a fixed estrus cycle like most mammals). Humans having two or three kids is actually RARE in agrarian or low-tech societies, because artificial birth control isn't easily available and they have to rely on abstinence, luck, etc. to prevent pregnancies.
We know they were born only a few years apart because their ages were stated. Two boys of thirteen and six and a girl of ten. (From the section involving Lord Rathekrel and the false betrothal.)
What really bugs me is the disjointed fashion the end of The Elvenbane is written in. We get Keman going back to the dragons to force them to help the halfbreeds, challenging his sister to a magic duel, and then as soon as Keoke says "Let the duel begin!" we jump to the aftermath, with Keman picking himself up after he lost. Talk about an Offscreen Moment of Awesome! Did they run out of room or something?
It's always bothered me how later books suddenly brought in iron as the Metal of Death for elves, when in the first book it's mentioned that an elven lord snapped a human's sword over his knee. Sure, sure, you could say that sword was bronze or something, but if iron's that lethal, then it should have come up before. The way it's treated later on implies that this has always been known.
Maybe the elf lord was wearing gloves? As long as he didn't hold the sword in his bare hands and didn't cut himself, there's no reason why he couldn't hold it...
The gladiators' weapons must be made of bronze, aluminum or something; there's no possible way that the elves would allow their oppressed servants to hold weapons that can not only easily kill the elves but play Tennis Boss with spells like levin-bolts. I don't think it's unreasonable for it not to have come up beforehand; the first thing the elves would do, whether they were just now discovering that iron could hurt them or knew it from the previous world, would be to get rid of all of it.
The pure-blooded humans have Psychic Powers, which are mostly suppressed by the collars they wear as slaves, but those magics and the similar versions expressed by the half-bloods are absolute anathema to the elves. So what's with the geas put on valued servants like assassins and the Change, not to mention Rena's fiddling with the alicorns' minds? If the elves can do mind-manipulation like that, why are the human mind-magics so awful?