The 6.3 mm cutter is deliberate to allow a snug fitting 6 mm panel. Even if you found a 5.5 mm bit you would struggle to fit your ply into it. For a painted finish MDF will yield a far better finish than ply. I would look at MRMDF which I have found to be an exact 6 mm. Bring a vernier to the yard to verify before purchasing - you really can't trust quoted tolerances. From memory the depth of the rebate for these cutters is about 15 mm -- you certainly don't need a 25 mm deep cutter.