As far as I have noticed (maybe I am wrong), you can't do the summon in those bosses which access is throw a room with spots and a ladder, no matter if it is a final boss or a 5 players one.
- Dream courts bosses
- Library bosses
- Urmahlullu boss
- Grave danger bosses (but the mini bosses which are on the respawn)
- The secret library bosses (but the mini bosses of falcon bastion that are on the respawn)
- Forgotten knowledge bosses
- Cults of tibia bosses
- Heart of destruction bosses
- Ferumbras ascension bosses
The rest which are rechable in respawns or throw teleports, you can do summons:
- Mini bosses of falcon bastion
- Mini bosses of cobra bastion
- Warzone bosses
- Were-cave of edron bosses
- Kroazur: Threatened Dreams Quest boss