Generally, only very closely related plants can be grafted--scion and
stock from the same species or genus. Rarely are intergeneric grafts
successful. Your tomato/eucalyptus scenario is probably impossible.

I don't think your method of "grafting" will work. A successful graft
requires fusion of two cambium layers, not penetration of the stock by
scion roots.

A good book on horticultural techniques--plant propagation--will
probably answer your questions.

M. Reed