What about , "as Goldsmith points out, “since we all eat. [FONT="]Of all the levels""[/FONT]
Yes, all these can start sentences.
What you are listing are dependent clauses that begin with a conjunction.
The typical word order would be:
"conjunction and its clause," "main clause."
Using the dependent clause to start the sentence gives variety to writing and sustains interest. It's boring to read too many plain declarative simple sentences in a row, but variation in sentence structure brightens up reading.
On the other hand, too frequent a use of this device is tiresome.
Here are some grammatical sentences that begin with the phrases you mention:
> "As Goldsmith points out, honesty really is the best policy."
> "Since we all eat, we should learn how food choices affect health."
> "Of all the levels I have mastered while gaming, this one is the most exciting."