I usually fly up at around 35,000 ft or so. When I call a tanker, It spawns down much lower, usually at around 13,000ft.
Poke around in $FGData/Nasal/tanker.nas and you ought to be able to modify the script to always spawn at 30.000 (or whatever altitude you like) - that's probably easier than rigging AI scenarios for you.
I'm guessing its just a case of changing the location of model in the <Model> tags, in the Demo xml, to the location of the model I would like to change it to? also, does the new Model I would like to change it to have to have a boom/drogue or not?
The AI scenario references a model file (that's an xml thingy) - that in turn references a 3d file (that's a path to a *.ac file). Boom location in the 3d file is pretty much irrelevant for what happens in-sim, I believe the offset of that is defined somehow in the AI scenario file (they're made to coincide in practice, but they don't really have to for this to work - refueling just checks your proximity to a certain moving reference location).
If you change the path to the *.ac file, you get a different 3d model loaded for the tanker. Basically the xml wrapper of a model contains stuff you do with the 3d model (animations, liveries,...), the *.ac file is the mesh itself. The xml file however needs to refer to objects which exist in the mesh, otherwise it ain't working.
So dependent on what you want, you may just get away with a text editor hacking the xml wrapper or you may need a 3d editing tool (blender, ac3d, sketchup) to change the mesh itself.