I've thought about this myself. Flooding the network with garbage would be a bad thing, so surely it will be attempted by those who are against it. One way to ensure that people aren't fooled into downloading garbage is to introduce a rating system, which allows the public to rate the servers they download from. The ratings would be shared peer-to-peer instead of residing on a central server; it'd be like word of mouth. Not perfect, but it would help.

As for matching the file size and name, Xolox would still reject the fakes because the data itself wouldn't match the originals. Xolox does block checking, like most other servents.