I don't think a big age gap is necessarily a problem, although it can be. t depends on the specific relationship, if it gives some imbalance, if you feel a cultural disconnect, etc.
I've had and seen relationships with a big age gap that were absolutely not hurt by it, and others with a smaller gap that were hurt, so there is really no rule for it.
About the kid, there could be issues having a child so early (both early in life, and early in a relationship), but many issues would be about being financially unstable, lacking support to raise the kid, etc, which would be less the case in a poly configuration, provided you get all the support from both your partners of course.
I think one issue would be the fact that she is pregnant. While I understand the idea of having children around the same time, it will also mean double the work. One advantage of poly is that when she has the kid, one person can take care of the kid while the other two still date. If there are two kids, it will be more tiring, as taking care of a newborn is hard enough.
So I guess my advice would be to be there for the child born from her and see it as practice from when you get your own, but maybe try having enough of the gap between the two that the relationship can recover from the birth before adding another one, with all the stress it creates. Plus you will get to see the both of use acting around a newborn, which will give you an idea of what to expect with your kid.