Nashville downtown near Wild Horse
I think Merchants downstairs would have a hard time finding a table for a party of 15. I've been at a table with about 8 people, and I think that would be their limit. Upstairs at Merchants is probably not as casual as what you're looking for (they've revamped recently and I haven't been there since, but they are meant to be more sedate upstairs than downstairs). Past Perfect is casual but not party-like. Broadway Brewhouse would probably suit your purposes, but stay away from Rippy's if you care about food at all -- it's pretty wretched there.
429 Broadway, Nashville, TN 37203
122 3rd Ave S, Nashville, TN 37201
The core downtown area isn't where the really good restaurants are. And I'm assuming by "fun" you mean sort of a party atmosphere. As noted, the Wildhorse itself and the new Merchants (downstairs) are decent options, although Merchants is not raucous. On 3rd south of Broadway is Past Perfect. Rippy's on the corner of Broadway and 5th is popular with tourists, never been myself. Between 3rd and 4th is Broadway Brewhouse.
A little further afield -- you'd want to cab it, but it's not far -- is the Flying Saucer in the Union Station Hotel (entrance on the side, not through the hotel). Broadway & 12th.