Could You tell me guys, how can i set the limits listed below?

1) I know i can limit number of processes per user, but what about background processes? When i allow user to launch 20 processes, then he will be able to start at least few screen sessions. I would like all users, who belong to specified group, to be allowed only to launch only one background process (using &, and starting it in screen). I know this is possible, but got actually no idea, how to set that.

2) I wonder if it is possible to specify max. number of cron jobs per user?

Thanks in advance!