This article on the Answers In Genesis website is a good introduction:
One short quote:
The Bible never uses the Hebrew term nephesh chayyah (living soul/creature) when referring to invertebrates, but it does when referring to humans and fish (Genesis 1:20; 2:7). Also, insects do not have the same sort of “blood” that vertebrates do, yet “the life of the flesh is in the blood” (Leviticus 17:11) (Sarfati 2004, p. 211). It is reasonable then to assume that the pre-Fall diet of animals could have included invertebrates.
Thus the distinction is between creatures with a certain kind of blood and the breath of life and those that do not. Creatures that lacked the breath of life and blood were subject to death, but that death has no spiritual component and is not the sort of death ushered in by the fall.
As to the second question about whether God created carnivores, I know of no scriptural evidence, but there is anecdotal evidence that carnivores can live on fruits and vegetables. Here are two articles, one about a lion that refused to eat meat and lived a long, healthy life, and the other about how crocodiles love to eat fruit.