"I should use a spline, draw my path then add the surface area of the geometry and turn it off?"
Exactly, this way, such surface will be used just for getting the correct position for items in the Spline Area.
"... when I hit the library button and load in say a small daisy type plant or something, it removes the original library and just loads in the Daisy instead of adding to it."
Importing any library will replace the current Forest object settings by stored ones (as besides of used geometry presets store all other Forest's data to reconstruct the distribution in the same way as saved).
You should create another Forest object and adjust these to complement each other or to add just wanted geometry to the existing object use the Geometry
> Add New Item
/ Add Multiple Custom Objects
option. All objects imported from libraries you can find all these under "forest_templates" layer. By default all objects inside of this layer are hidden and frozen. The object name will be _forest_library_*******
. Any of these objects can be manipulated like any other 3ds Max's object - moved to other layer, copied etc.
Hope that helps.