Starter mix is finer textured thus making more contact with the seeds (moisture) for better germination. Stay with the starter mix.

I use seed starting mix(Promix) for all my germination.

Here is a link that might be useful: Gardening by the Seat of my Pants

I'm way new at this, obviously. It never occured to me I might have to pot-up my starts. I used peat pellets for all my starts, lettuce, thyme, chives, onions, brussels sprouts are up so far. Assuming my last frost date is toward the end of April, I should be planting the lettuce in March, will I have to pot them up? How do I know that I need to?

If it becomes rootbound then it it should be transplanted. If you can spare 2 weeks then yes move to a larger pot, if not leave them be. Timing is everything if the roots don't become bound in time for final transplant then the roots can become too damages to repair themselves. Trial and error is the best because each plant grows at a different rate. One time for a slow growing plant can be different .