seed away! I find that clover seed is a great addition. The low plants and flowers are nicer than tall grass where you can't mow.
Celery is not "bad" as such, but is also not "good". It's just an empty food that is little more than fiber and water. If you use it, just don't use it often. Iceberg is the same. One problem beyond it's emptiness is that some turtles will decide to only eat it, if it's provided often enough. I find that tossing it in every month or so causes no problem. Celery and iceberg are also not eaten any more quickly than any other vegetable in my pens. My boxies definately have a preference for meats first, fruits second, and veggies last.