With the nitrogen cycle... read this:
http://www.fishlore.com/NitrogenCycle
Depending on how old youra tank is, and how long you've had fish there, it might no longer be "a problem" but the nitrogen cycle is something vital to know, because when something goes wrong in a tank id say 7 out of 10 times it has to do with something related to the nitrogen cycle.
and yes, I know what a biorb is. it is just a ridiculosly expensive fish bowl in which you can barely keep a couple of small fish due to its shape and size. (although I have to admit it looks cute).
regarding the overstocking, you have only two options:
1) giving back to the store the hatchetfish and the ghost catfishes. In which case you'd be left with three mollies adding up to 9 inches of adult fish i an 8 gallon, still, one inch overstocked but not too bad.
2) getting yourself a bigger tank (aside of the biorb) to put two schools of fish, 5 ghost catfishes and 5 marbled hatchets. which would be a bare minimum of 30 gallons.
You also need to know what will you do if your mollie has more babies. livebearers can have from 1- 50+ fry. Do you have any local fish store that might take them in case she has more?
-Alessa.