The pre-verbal particles fe, mi are sometimes used to mark a verb as being affirmative. Both cause a soft mutation of the verb. Unless you are with people who are using them, it is easiest to leave them out - just be aware that some people do use them.
Am I right in thinking that "mi wna i wisgo sgert", "bydda i'n gwisgo sgert" and "gwna i wisgo sgert" all mean the same thing?
As explained above, the mi/fe particle is optional. It makes no difference at at all to the meaning of a positive statement.
- Gwna i wisgo... means 'I will wear...'
- Bydda i'n gwisgo means both 'I will wear...' and 'I will be wearing...'
A slight difference, that is all, and in most situations it can be ignored.