Lots of good advice here, not much to add except that the cause of your nitrates could be many things, but I bet overstocking would have attributed. If I were you I would pick either 1 Chromis or 1 Clown, and then pick one smaller fish like a Goby, but be aware that the Chromis will turn into an @$$ when he gets a bit bigger so could very well kill a smaller Goby. Both Chromis and Clowns grow fairly large for a 10g, so having one of each would be too much imo. To have both of those fish I would suggest a 20g minimum. Or take the Chromis back and get a Clown instead??
Oh, and it will take a good 8-12 months for the dry base rock to become fully functional live rock provided the base rock is high quality porous dried live rock from the ocean, and not a farmed rock from the land like CaribSea Reef Rock which will never become fully functional live rock in regards to nitrate processing.