From the examples of litotuko versus litkovrilo, and from the listing of both lignoglavo and lignglavo here I concluded that the the inclusion of -o- between two connected roots is at the discretion of the author, determined by what's easy to pronounce. Is that true or are there more deterministic rules?
A tightly related question is what grammatical affixes can be left out. In this answer I noticed flavĉapela leaves out an -a- or perhaps even an -e- so aparently this follows different restrictions than those of poetic elision (rule 16). Are there examples for either of -o- / -a- / -e- / -i- excluded? Or can I always do it, as long as I don't create a tongue twister?