A good idea, if using multiple populations of the same object, is to visit the internal network of your first population, cut(ctrl+x) the object node and then paste it to your main network view(this is not necessary but it makes your network more easy to understand than leaving the first object inside). Now, select this single object as the 'object maker' of each population which will use it. The population node has controls that can manipulate this object according to what's required of the pop' it's plugged into.
This saves lots of memory, especially if using large filesize objects with lots of large texture maps, say. The object only needs loaded once and you still have all your populations.
Instead of creating new populations that will contain another object, just duplicate(ctrl+D) the empty pop' node and assign the 'object maker', 'planet', 'terrain shader' and edit its settings to where the new population will be, scales, size etc.