It depends on the specification and the analyzer, but according to the CommonMark specification:
An ordered list marker is a sequence of one of several digits (0-9) followed by either a. symbol or a) symbol. "
http://spec.commonmark.org/0.18/#ordered-list-marker
Therefore, I would avoid any other style, since it would hardly be compatible with any parser.
If you want to use sub-lists, you can backtrack from them. This, for example, should work:
1. One
1. sub 1
2. sub 2
2. Two
3. Three
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