Minecraft – Spawn the Ender Dragon even if I broke it on creative

No, there is no level range that strongholds must spawn at. If you take a look at the page on Minecraft Wiki, you can see that strongholds can spawn on any level in the world except for partially or fully above ground. In other words, they must always be below ground, but apart from that there is no limit.

The use of spawn eggs ignores most of the checks for whether something should be able to spawn somewhere. In particular, no check is made for amount of mobs already spawned. You can use spawn eggs to create hundreds, thousands, or even millions of mobs in an area; the only limit will be the processing power and memory needed to handle them.