The OSM data suffers from well-known problems with forests at low zooms caused by CanVec imports in Canada. Although outside Canada the data is generally good, the advantages of OSM data don't really apply at very low zooms, and another data source would probably be better.
In case needed, Kartotherian has a module to combine multiple sources based on zoom - e.g. data for zooms 0..4 can be coming from one source, and 5+ from the other
Not really. In this context, source means the origin of the data. It would still be loaded into the same database and accessed through the same queries.
NE doesn't have forests, but I adjusted zooms and the problems aren't as bad. I'd still like to find an alternative source for forests, but as this issue was specifically about NE I'm closing it.