Yes, that is of course to get the most value out of the DP. The resturaunts that are more expensive take 2 credits. These are easy to see if you look at the dining plan brochure on the WDW site.
Also it's good to look at menus on www.allears.net to get an idea of what you would like to eat to make ADR's.
Yes, you can order any entree. The extremely few limitations are found in the FAQ sticky at the top of the forum.
With respect to previous poster, signature (2 credit) meals generally aren't the best "value" use of credits, but it's not like you're losing money on them, either.
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