... to rough-in for the bathroom I'm putting in. my questions are this:

1) Do I have to use Cast or can I use PVC for the stuff I'm putting in?
2) With the layout and location of things, it's going to end up where my bathroom sink and wall will be directly over the main line in the floor. Can I run a "y" off the main vertically instead of laying it horizontally?

Thanks for you help guys! I appreciate it.

I would go with PVC. Follow directions on the can of cement, clean the pipe, use primer, and even if it is your first project, you will get a lasting job you can cement over in confidence. Cast iron is for people with skill acquired under the eye of an experienced person.

A vertical Y plus a 45 elbow will mean fewer elbows to clog and easier clean out if it does. Listen to Tom, he may know about a problem I don't.

... do you think that I will have a problem with fitting/sealing the new PVC into the cast that is already there? Is there a benefit to cast over PVC?

There are boots and Ferrco? Donuts made to join cast to PVC. Follow directions and they should work fine. Both cast and PVC will last a long time. Cast will stand more mechanical abuse than PVC, and there may even be places code forbids PVC. Bans on cheap, easy to work with materials limit who can do do the work.

... *winking as if having seizers* of COURSE I'm going to have the city inspect my job.*winking one more time just in case* Is anyone going to get introuble if I put PVC in there? Where can I get the PVC? I know that I have to go to a plummer for cast, is it the same for plastic?

...*winking as if having seizers* of COURSE I'm going to have the city inspect my job.*winking one more time just in case* Is anyone going to get introuble if I put PVC in there? Where can I get the PVC? I know that I have to go to a plummer for cast, is it the same for plastic?
... hardware store (Ace, Lowe's, Home Depot, etc.) will have PVC and ABS pipe and fittings.