I have no idea about their opening date, but if I had to guess, I'd say they'll open before everything is finished, just like Bioparque do Rio. If there are expansions to happen, they might take place with the zoo and park already working.
About the enclosures near the gorilla exhibit, they are supposed to be for cheetah and spotted hyena, according to the map they have previously released. That lighthouse enclosure near the rhinos is supposed to hold white pelicans, and one of the aviaries in front is already holding flamingos, afaik.
Talking about the gorilla exhibit, there's no such a thing as a perfect size. Theoretically, the bigger it is, the better. Of course, it's a zoo, so animals should be visible at some point, even though they should have spots to hide whenever they want. If you check our enclosure for said animals here in Belo Horizonte, it's not extremely big (2000m2) yet it's pretty suitable for them. So it is that they have bred four times in less than ten years together. Enrichment and vegetation has a huge role in my opinion. Also, we're not sure the Animalia is indeed gonna keep gorillas; these are very difficult animals to obtain and they haven't contacted BH zoo so far. Let's wait and see...
On the regard of elephants, I have heard they were initially the main focus of the Animalia park (so it was that it was to be named as "Parque dos elefantes" until a certain point) but they no longer intend to keep them, at least not for now. They are about to receive five giraffes, but I have no idea when that would happen; and about the pandas, I have absolutely no information.