"I saw you yesterday."
Translation:Gwelais i chi ddoe.
Two different ways of saying the same thing, really. gwnes i'ch gweld chi... is very common in the spoken colloquial language, but gwelais i chi... is also commonly used in the spoken and written language. You will hear and see both - it is more a matter of personal style than anything else.
No, fi is not used like that.
The basic form is Gwelais i .... Some people in some areas and in some levels of formality sometimes put Fe or Mi in front of a positive statement - this cause a soft mutation of the verb:
- Fe/Mi welais...
In the modern language this Fe/Mi has no connection with the person doing whatever the verb is describing - I, you, they, she, Fred, the dog, ...