For 5/32" rivets you can stay 1.5D (D being diameter) or more from the edge. Ideally it's best make the new rivets tie up in some way with the existing rivet locations, maybe look horizontally and pick some idea up from them. Airstream rivets are spaced crazily far apart but it seems to work so just measure on the your trailer and duplicate. Tip of the day is to layout your holes and drill to #40 or #30 and when you are happy upsize and then draw the outline of the patch and cut down to size.
I would suggest you squeeze your sealant of choice, say Vulkem, on a scrap sheet and roll it over the entire patch with a small roller then install. You do not want the patch to be in contact with the skin without something in-between, it allows moisture to hide and corrode your trailer. Slathering heavy beads of sealant is just a mess really, it's OK on the edge of a skin or patch but too much goop just makes the riveting harder and will deform the skin a bit too.
Hope that helps.
1960 Sovereign 33' Pacific Railroad Custom