I've studied Ruth, and I don't see the correlation between the Ruth of the Bible and David's Mom. (other than both Ruth's were Ancesters of a King David.)
Now David, I can see being loosely based off of King David from the Bible. Both were shepherds, Both had to defeat giant foes, Biblical King David's first wife was Abigail. And ultimately, both became kings of their realms. (and I hope that is as far as they take it).
Having said that, if David is intentionally based on the Biblical David, that troubles me. Not only from the standpoint that King David was very much a real person (more so than most of the other FT characters). But also the implied comparison of the Bible to a Fairy Tale. That is something that I personally find offensive. Maybe they did not intend that, but it's there none the less.
OK, I'll put my soapbox up now. Carry on with the discussion. :)