When you have a working Telugu font, could you share it with us the improve the support in Glyphs. Starting with the glyph set in the sidebar. And then automatic OpenType features.
For the name of the not encoded glyphs, there are similar script to look at. e.g. Oriya: There we have the following glyphs: e.g. ka-oriya, ka-oriya.base and ka-oriya.below. (the “.base” variant is only needed if the shape needs to change when there is something below (e.g. moving certain feature up a bit). It would be good if you could follow that pattern (unless there is a good reason to do otherwise).