Don’t tag for the renderer. See http://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer
If Dartmouth is a city, then it should be tagged as place=city. If it is something else, it should be tagged as such, eg place=town.
It is true that if you have two cities close together, maybe only one name will be shown when you zoom out. And it won’t necessarily be the biggest / most important one that is shown. This is a problem with the rendering software, I’m not sure if there is any way to specify which one is more important.
You could tag the population of each, if you have a suitable (non-copyright) source for that. Or map the outline of the city as an area. I don’t know if this will make much difference to rendering.
What do you mean by data from connect.garmin.com?
If you mean the maps, they are from Google so are copyrighted.
If you mean the GPS tracks from individual activities, they are presumably copyrighted by the user that created and uploaded them (and they might be copied from other maps anyway). So can’t be used without permission of that specific user. It may also be forbidden by the Garmin Connect terms and conditions, I’ve not checked.