I do not believe there is anything i75 or above that *can* be Materia Melded; its all Mythology Token bought gear, or Allagan gear dropped in the Binding Coil of Bahamut.

I want to say that Converting an item of level 25 or higher can result in a Tier 2 Materia, 35 and higher for Tier 3 Materia, and 45 and higher for Tier 4. The lower the item is within that range, however, the more likely you are to get the lower level rather than the higher level (so, an item level 28 is more likely to produce Tier 1 than Tier 2, and an item level 45 is more likely to produce Tier 3 than Tier 4).