seachells wrote: 1. do severums mate with other types of cichlids and if so, is there a type that this usually happens with?
Severums are known to cross with other species of severums. So, Heros species do cross with other Heros species, though it is probably not all that common. They can cross with very closely related species, all in the same genus, Heros. Now, I've never heard or encountered any claims of them crossing with fairly closely related species (say for example chocolate cichlid or festivum) though it is conceivable that they could produce offspring with these crosses(??). I think you don't encounter these types of crosses too often because none of these fishes are inclined to hybridize, especially outside of their genus. There are a few claims of severum X convict crosses on the internet. The general thought is that they are too distant to produce offspring (though for the moment they are still considered to be of the same tribe, though I would think with the modern propensity of excessive splitting, this might change in the near future). One thread showed a supposed offspring that looked the part, though it really could just have been a deep bodied convict with an odd shape as convicts are highly variable.
seachells wrote: 2. why weren't the eggs fertilized if the severum is guarding them?
In my opinion, it is very likely you are mistaken on the sex of your severum. If your sexing by the pattern on the gill plate, bear in mind it is not that unusual for females to sometimes have some worming pattern. Not always a 100% reliable way to sex many severums.
First of all, it is very unusual for male substrate spawners to tend eggs (yeah, there are a few exceptions like jewel cichlids where the males may tend the eggs as much as the females.) Generally, tending eggs are the exclusive domain of the female while the male patrols the perimeter of the territory. Fry is different as both parents may be involved just as much. Now with unusual circumstances, occasionally you might see a male take over egg tending duties (eg. domestic dispute where the male chases the female off very early or an aggressive female only interested in defending the border and male takes over because she is neglecting her job.) So in order for your severum to be a male guarding another species eggs, he would have had to pair up with another species (fairly rare for severums), then shortly after doing the mating act have a serious domestic dispute and chase the female off. Much, much more likely your severum is a female that laid eggs in the absence of a male. That is nothing too unusual for most substrate spawners.