Black Skirted Tetras
 

I just picked up 5 more today, their still little babies
But the 1 i already had is what i think to be full grown...

SO my questions are

1. How do you sex them?
2. How do i encourage them to spawn?

Is hard sometimes(most times) to sex some of the Tetra species. Females are often a bit fatter than males and a bit larger.
In any event,,with the other fishes listed under your aquariums,,the eggs from spawning would have little chance of surviving for the other fishes, as well as the tetra's themselves,, would eat them.
You would need to set up separate tank, and place a few females or a pair in the tank and either provide dense cover in the way of plant's for fry to hide in or,,remove the parent's after the eggs were laid and feed the fry with suitable foods.(Micro foods,infusoria)

Ive done the whole seperate tank thing for danios. took the parents out next day, ended up with 60 fry until 3 weeks later i had an ich outbreak in all my tanks and lost all the babies

also i have once found a baby danio fry in my tank before, i tried catching it to put in the net breeder but squished it :(

but if i put them in a seperate tank, how do i know when the female and male are old enough to spawn, like i said the new ones are babies

here are some pictures