The indices guaranteed to be in crescent order, right? All they can do is skip some indices?
I'm not sure what you mean by "crescent order" (I'm guessing this is a typo for ascending order? or is it something else?).
In any case, the order does not matter at all. The sample default file just proceeds in ascending order (and skips unrelated characters) purely for convenience, but it is simply a list of matching pairs that can be provided in any order you want, one pair per line.
The "indices start from 0" part is referring to the source bitmap's layout and how to reference each glyph.
