Heatran, yes. Though you can go to where it's caught, you need the magma stone from route 18. Cresellia, yes. You need to catch it on Skyarrow bridge, even though you can get the lunar wing before the E4. So, on both, sadly,YES.
Source: Dat Knowlege
Yes. They are only available post-game.
How to change your username
How to add a user avatar
Why was my post hidden?
The points system
Flags vs Downvotes
Type a Pokemon name, type, move or ability to see instant results.