Yes, of course, as it is necessary in order to balance the ecosystem. If the grass grows too slowly, the creatures will not have enough food and will die. Instead, if the grass grows too fast, there will be an overpopulation and there won't be the conditions for the natural selection. Therefore, it is possible to create favourable and unfavourable worlds changing few parameters.
In the pictures every circle represents a single creature. The dimension of the circles can represents different characteristics of the creature (energy, speed, bigness, ...). In the pictures on the website circles size represents the energy of the creature, which means that the bigger the creatures are, the more they have eaten just before.
Yes, the program is ready, nevertheless it is not usable for analyses because it is quite difficult to balance. In fact, the ecosystem with both herbivores and carnivores has to be balanced perfectly otherwise both the species will die.
No, they can be thought as any species of herbivore.