True to all that has been said.
Casting multiples in a gravity feed mold is..... troublesome unless you know what your doing. I would make a one cavity mold and just concentrate on making as many GOOD casts as I could. Its easier to make sure ONE is filled well that with 6..... multiples are best done with a setup using vacuum and pressure.
As far as Recasting.......
Now personally I dont give two shakes of a stick what you do with your models as long as you keep it to yourself. You cannot sell give trade or ANYTHING recast parts legally and if you do so you are a criminal in the eyes of the American law. If you are recasting miniatures for profit you are morally wrong too. NOW that being said I have some models in my armies that I have recast parts on. sometimes entire models are recast. But these are MINE and I am not selling nor giving them away.
If your wanting to do this know that there are some serious repercussions and you have been warned. Im no Lawyer but look at GWs track history on IP enforcement and Swing onto Bartertown and look into the mail fraud and IP law sections there.... Linrandir is a personal friend of mine and he tells me things that are just not fun for those who get caught.....
/end rant
NOW posting a picture will help us understand whats going on and help you out. The BEST thing I can say about gravity cast ANYTHING is fill from the bottom, and vent vent vent anything going up.... I have googled some pictures to help....
Simple vents with parts
Actual mold with vents- notice how the pour is low and vents to high
Very complicated mold with vents.... Again notice how the pour starts low and vents high