No, actually is not possible to select trees by ID (we would include this option for the next version).
As workaround, you can use Maxscript to modify the trees. This sample script set width=5,height=10 for all trees with geometry ID=2 for the selected Forest object:
for n = 0 to $.trees.count() do
id = $.trees.getGeomID n
if id == 2 do ($.trees.setWidth n 5; $.trees.setHeight n 10)
About your second question, when you create a tree on Custom Edition mode, the plugin chooses a random ID based on the probabilities defined on the geometry list. If you want to randomize all IDs, then use the "Randomize Geometry" button... what do you want to do exactly ?