"Maybe the hostile mob spawning area is identical with the slime spawning area (slime chunks)"
I believe this is correct, although I have not conclusively tested.
Mark, slimes spawn in slime chunks between Y 0 and 39, however hostile mobs will spawn from Y 0 to 255.
Slime blocks placed in a slime chunk at Y63 (sea level) will not spawn slimes but will spawn hostile mobs. However, if place between 0 and 39 we should find both slimes and hostile mobs spawn.
If slime blocks are placed outside of slime chunks it is my understanding that no hostile mobs will spawn.
As I have said, this is not conclusively tested, but I believe it may be 'as intended'.
Issue exists on my 1.8 server, chunks were originally generated in 1.7.9.
Seed: 26551365887039689
Coords: -333 32 -225
If seed is used in 1.8, a temple is present.
Not invalid, but duplicate of MC-30864