Yep. It's not the best for them (like any other lettuce), but it's fine as part of a varied diet. Other than having it there for grazing, I only really trim it (and the Mesclun) for salad use maybe once a week, and even then it's not much since they haven't grown very much yet.
Re: Soil, I've been using a mix of cheap top soil and some "moisture control" potting soil (found it in the garage after figuring out what total crap the top soil is). It's mostly the moisture control, mixed with some top soil, and a couple inches of just top soil on top to keep them away from the not-beardie-friendly ingredients in the potting soil (i.e. perlite). I don't use any extra chemicals. The only fertilization is the manure and fertilizer pellets in the soil mix... Not enough chemical, I don't think, to cause any issues for the dragons, but still enough to give the plants a little kick in the growth department.
If you're really worried about chemicals or things like that, Miracle Grow sells an Organic Potting Mix that's pretty good. It doesn't have any "extras" in it (not even perlite). My spider plant has been doing really well in it for a year and a half now--even grew back from being eaten to the ground twice (before I figured out tortoise and beardie enclosures make very hostile environments to yummy green stuff... lol).
The entire pansy plant is edible. My girls actually do enjoy the taste of the whole plant... They wiped out at least one of my pansies in the garden, little snots. I got all my pansies and violas from the local nursery and Walmart. I'm impatient, so I didn't take any time to let them "grow out" possible chemicals, but the beardies don't get nearly enough of the pansies to cause problems. I would
never recommend my nonchalance with a baby/juvenile beardie, of course.
It helps (or, you could argue, doesn't help at all) that I used to work in a nursery. We never added chemicals to the plants, even the plants we started from seed ourselves or when repotting plants at the facility. Whatever they came from the wholesaler with and whatever was already in the soil was it, and it seems to me that most nurseries would follow this mindset, as well... After all, the plants do fine without them, and all those chemicals cost money. As well, pansies and violas are actually used a lot in salads (for people)... I like to think nurseries/garden centers realize that and treat them more like veggies. But I don't have any evidence to support that. =P
Starting your own from seeds is time-consuming, but definitely the best way to ensure the plants you grow for your beardies are safe.