I expect they are talking about large amounts of things. Even peals, tops and other kitchen scraps will heat up given enough material in the right conditions. Your lettuce should be just fine for the worms so long as you are not filling the bin with too much of it.

Since you are just starting a new bin you will probably only be feeding about a handful at a time at first anyway so you won't have to worry too much about compost heating. As your worms get down to eating and increasing their population, you will want to leave an area of the bin with no food so in case stuff does heat up, the worms will have a cool place to go.

What size is your bin? How many worms?

Thanks for your advice on this. We purchased a "Can-O-Worms" bin and have ordered 1000 Red Wigglers to start out with.

berrypatch,
You will be feeding small amounts to your wigglers for a while so you are not likely to make the bin hot. Just bury a handful of food in a different spot each time you feed. Wait till the previous meal is gone before feeding again. Adding a little new bedding every once in a while until you are ready to move up to the next level.

Good luck with your can-o-worms