How to introduce predators to a simulated ecosystem?
What you're trying to do sounds really cool. I feel that you can do the following:

1) Increase reproductive period of predators

2) Increase prey to predator ratio (see Lotka-Volterra equation)

3) Make the reproductive speed/ ability of an individual to be in relation with the number of energy 'points'

What kind of set-up are you working with, a unicellular model or a larger scale?

Maybe you could start with a solely herbivorous setup, but give all individuals the ability to harm/sap a small amount of energy out of another individual.

Is there some way you could send me your project? I want to have a look at it