I have read that most Eucalyptus chrysalls easily - is this true - or very species specific?
There are over 700 species of Eucalypt and yes some are brittle and will crystal readily. Otis has given you the name a number of trees that you can make good bows out of.
Has anyone made board bows out of it?
A number of the woods Otis mentioned are available in Australia as floor boards or decking boards and yes people have made plenty of board bows out of them.
Must it be backed?
Not necessarily, depends on the grain and annual ring orientation plus the weight of bow you are making. Same as any western/American tree.
What diameter tree to you start with for a self bow?
Its wood just like that in the West, so same rules apply, some have made bows out of 3inch saplings others from larger trees, it also depends on the type of bow you are after.
What is the best way to season it from scratch?
Split into staves, seal the ends leave the bark on or remove it and seal the back. some species check terribly no matter what you do.
Craig.