I understand this ruling, which is pretty normal for Tournaments:
•Models must be played on the bases provided with them.
In my specific case, there are quite a number of Tyranid Special Characters that have never had a model created for them. For some of these monsters, it seems fairly straightforward: The Doom of Malantai would have a Zoanthrope Base, the Tyranid Prime would have a Warrior Base.
There are also some existing models that used to be released with smalled bases: The old Hive Tyrants (Square base, and warrior size base) would up-size to the the current Hive Tyrant large base.
For models like the Harpy, Parasite of Mortrex; I'm not sure about. The flavour for the harpy says it's a lighter Monstrous creature, that is related to the Trygon. Would it use a Trygon base? or a Hive Tyrant base? Is the Parasite of Mortrex a Gargoyle base? or a Warrior sized base?
I've never seen any official ruling on this. Since these models are all custom conversions, it seems somewhat open to interpretation.
What size base should I be using for models where the base size is not clearly defined?