I have City Navigator, old Topo 100k, Topo 24k southwest plus lots of custom kmz maps loaded on my Montana. No problems.
I personally think the DVDs are the way to go. But I only use one GPS at a time for several years.
The main advantage of the topos is that they often show minor roads that do not appear in CN. So I tend to use them more for trip planning. And the old 100k topos show often show different roads than the 24k version.
I use the newer Mapinstall program to install maps onto the Montana. I used to use Mapsource for my older units.
If you are new to this, I would recommend just installing a few map tiles to figure out the process and see how this goes. Once you get some confidence go ahead and install everything.