We call out the limitation in various compatibility problems with builds earlier than 2.6.5, however I don't think this goes far enough. Due to
SERVER-17087, if a 2.6.5 (or 2.6.6, or 2.6.7) member is re-sync'ed from a 3.0 member running WiredTiger, the initial sync might report success but be fatally broken (it will have copied exactly one instruction in this circumstance, almost everything else is not copied). The initial sync has a good chance of total (and noisy) failure though.
I believe the first working version of 2.6 that can sync from a 3.0 member, regardless of options, is 2.6.8.