hybrids between related species are not uncommon in the wild and in the garden. the pauciflora group (including deubeuavielii) of subgenus eucalyptus will not combine with the gunnii group. paurciflora could potentially breed with the related ashes (e. regnans, delegatensis, oblique, etc. ) and peppermints (coccifera, pulchilla), nitida). gunnii has obviously bred with the related species you mentioned and probably many others within the subgenus symphomyrtus. the problem IMHO is not so much finding compatable partneres but if you can get both frost hardiness and (in certain cases) approprtiate flower colors---leucoxylon is not an especially frost hardy species and combining it with a hardy species will not necessarily guarantee increased hardiness in the first generation of progeny. most likely you are looking at going through several generations of crossing and recrossing and testing to locate and then "stabilize" a hybrid with reliable chareactersitc. however, you never know what you may end up with and it could/should be fun. good luck.