Seperating the stages is definatly a good idea. The worms and beetles will chew on the pupa if you leave them in the same container. Here are the time frames for each stage

Egg: 3-4 weeks
Worm: 8-10 weeks
Pupa: 1-3 weeks (in my experience 1-2 weeks)
Adult: 3-4 weeks

So when you purchase some mealworms from the store, they will probably already be a few weeks old. I started getting pupa about a month or so after getting the worms. I started out with 100 worms and I now have 8 beetles and several pupae. I'll be checking today for more pupa.

As for food, I feed mine a mixture of things. Uncooked oatmeal (the kind for the cylinder container), bran flakes, corn flakes, cherrios, Nutro dry cat food, potatoes, lettuce, strawberries, cucumber, ect.

After a few weeks you'll notice that the bottom of your worm container will gather a dust like substance. This is poop and needs to be dumped. If you are dumping food and poo from an adult container, its best to dump it into another container as it might contain eggs and baby worms.

Its takes a few months to get a colony started, but mine is on its way to producing me worms.