I don't think palaces and trading posts will take up regular holding spaces. Else a republican player could fill all neighbouring holding slots with trading posts, and the only thing the neighbour can do about it is fighting country claim wars every 10 or so years.
Yeah, while I have no information myself, either, I wholeheartedly agree with GothicEmperor (what a great, awesome nickname, by the way!). I think we can safely assume trading posts won't take up holding slots.
If you look at it this way, for example the city of Galata in the very "county" of Constantinople is basically an Italian colony (Genoa, Venice etc.). In later starts of the game, it may actually have an Italian Catholic mayor! (Such as in my Polish AAR.) A trade outpost would have to be a veritable city in its own right if it were to take a full holding slot the same way Galata does. On the other hand, they might do some reworking and actually reform Galata into an outpost. But even so, you would necessarily have a full city in that holding, not a minor EU3-style thing (+1 income from the province IIRC) actually taking a whole big slot from you.
I speculate that trading posts could be buildings erectable only in cities (no castles or bishoprics with such outposts) or they could attach in some other way to the county. In fact, possibly an additional holding slot but one only usable for the purpose of having a trading post there and not something else.
And as far as I can tell, palaces are not holdings. Venice in particular will need to keep the existing baronies and bishoprics, while I imagine it will have a hefty couple of palaces for the leading families within the city proper. Those palaces, I think, will generally serve and interest only the specific patrician family and not be a normal taxable holding that would be of interest to the liege. But this is just speculation.
