~1 lb per gallon should give you decent flavor with mangoes. Up it to 1.5 ish if you really want more flavor. My procedure for sanitizing fruit with peels is as follows:
-Wash fruit with soap and water
-dip fruit in Starsan solution (mixed per manufacturer instructions)
-peel fruit on sanitized cutting board
-cut fruit into chunks
-put fruit chunks into zip-lock with small amount of starsan solution (~1oz)
-freeze fruit for 48hrs
-thaw and dump into carboy
The freezing helps break down the cell walls in the fruit and will get more juice out. The sanitizing procedures described are one step below boiling the fruit, which risks creating pectin haze. A small amount of Starsan doesn't pose a risk to your beer in small amounts, the yeast will actually use the phosphates in the Starsan for growth. I have two plum beers on tap that I made using this method (without peeling of course) and they have not become infected after 6+ weeks in bottles.
Keeping the mead on fruit for 2-4 weeks should be about right, but I have little experience with meads, so perhaps someone else would know better. I take my beers off the fruit once the color has been extracted from the fruit and the fruit has sunk to the bottom. If it was a lambic or dark beer you might leave it on for 4+ weeks.
