is it correct that,
if the verb ends with "e", je and il have the same form, and tu has an extra "s".
if the verb ends with "x/s", je and tu have the same form, and il has "t" instead
It's perfectly okay, i suppose. Listen it on Google Translate: http://translate.google.hu/#fr/en/je%20veux%20manger%20du%20riz
I think there's a difference between the use of Présent Indicatif (je veux manger du riz) and Present Conditionnel (je voudrais manger du riz).
The latter one is more polite, in a formal situation I'd use that one (and by formal situation I mean any situation where you normally wouldn't "insist" something, you'd rather "ask" something).
I'm sure Duo's moderators know this as well, but conditional hasn't been taught yet, that's why the program is saying this.