I have a mesos / marathon system, and it is working well for the most part. There are upwards of 20 processes running, most of them using only part of a CPU. However, sometimes (especially during development), a process will spin up and start using as much CPU as is available. I can see on my system monitor that there is a pegged CPU, but I can't tell what marathon process is causing it.
Is there a monitor app showing CPU usage for marathon jobs? Something that shows it over time. This would also help with understanding scaling and CPU requirements. Tracking memory usage would be good, but secondary to CPU.
It seems that you haven't configured any isolation mechanism on your agent (slave) nodes. mesos-slave comes with an --isolation flag that defaults to posix/cpu,posix/mem. Which means isolation at process level (pretty much no isolation at all). Using cgroups/cpu,cgroups/mem isolation will ensure that given task will be killed by kernel if exceeds given memory limit. Memory is a hard constraint that can be easily enforced.
Restricting CPU is more complicated. If you have machine that offers 8 CPU cores to Mesos and each of your tasks is set to require cpu=2.0, you'll be able run there at most 4 tasks. That's easy, but at given moment any of your 4 tasks might be able to utilize all idle cores. In case some of your jobs is misbehaving, it might affect other jobs running on the same machine. For restricting CPU utilization see Completely Fair Scheduler (or related question How to understand CPU allocation in Mesos? for more details).
Regarding monitoring there are many possibilities available, choose an option that suits your requirements. You can combine many of the solutions, some are open-source other enterprise level solutions (in random order):