Hybrids: That has been discussed many times in the past (for races, not castes, although the standard caste case is very simple and exists in DF: offspring of the male caste and the female caste randomly belong to either parent caste, while same caste creatures won't produce offspring. More complex caste societies again produce offspring that belong to one of the castes, not hybrids). It's a matter of descriptions as well as determining how to determining the characteristics of the offspring for creatures with the same basic body plan (flipping a coin for one or the other as a base for a feature, or interpolate, or a random value in between the two; base of the individuals or their races, or outside the range of either, etc?). The more difficult case is called the Centaur problem here, i.e. how to mash together different body plans into a new creature. Both cases are intended to be addressed at some time in the future, and at least the first problem probably has to be solved during the Myth & Magic arc when procedurally generated races are introduced (not exactly the same, but the logic is probably going to be much the same). Whether the Centaur problem itself will be tackled remains to see (it's also possible to generate weird critters using a number of templates).
The life span of offspring of two races can be determined to be that of either parent or somewhere in between. When one parent is immune to aging the "in between" would have to first realize that the offspring is now aging, and so assign some max value for how old an aging creature can become.
It can also be noted that offspring parameters may not lie within the span of that of the parents. As far as I understand, ligers and tigrons are larger than either parent species, for instance.