I've tried both programs but prefer IMGfromGPX - but you have to be on the internet when you use it.
You can name the map overlay when you build it, and you have to give each map overlay a unique map number.
The tracks (or routes, IMGfromGPX will also convert routes) will be the color you had selected in Mapsource unless you over ride it at compile time. So you could color code your main route in one color, and alternate routes in other colors depending on the difficulty.
Just to test things out I built a really big overlay - TAT, TCAT, TET, CDR, PCQ - which came in at 1.4mb!
I've loaded overlays on NUVI's and they worked fine.... which is a good deal since you can't do much else with them.