I tried both natural caves and different levels of branch-mining. There are 6-7 players on my server and none of them found any tin or silver outside Nether. There seems to only be vanilla ores outside The Nether.
EDIT: I also have a 60x60 quarry that doesn't find any other than Vanilla ores.
EDIT2: Never mind. I just figured it out. (started a vanilla server and switched to tekkit with the map already generated.)