iOS development question

Having additional scenes will not cause problems with performance. Shoving everything into one scene may, depending on how much memory those assets take up, since you're trying to load up more or less all the game's assets all at once.

Depending on what your target hardware is, you may or may not have any issues with this, though. If you're targeting a newer generation iPad, and your assets are fairly limited in scope (ie. you don't have fifty billion polygons/draw-calls among all the levels), chances are you won't have an issue.

However, if you're targeting say, the iPhone 4S, or even the iPhone5, you need to keep in mind that the processors and ram are quite limited when compared to those of even the cheapest of modern laptops (most modern laptops have around 4 gigs of ram, minimum, the iPhone 4S has 512MB ram, and the iPhone 5 has 1 gig). Mobile device processing power is fairly limited, as well, as it requires a lot of power to run, and more powerful processors typically run hotter, as well. Both of these factors don't bode well with mobile devices, as there's not really any breathing room for any of the parts, and nobody wants to meld their phone to their face when talking to someone.

So, my suggestion would be to not load all your assets at once. Your primary concern for performance should be restricted to two things:

1. Restrict and optimize draw calls, whenever possible. Try your best to use mobile-friendly shaders ONLY, and don't have too many materials or objects being rendered in any particular frame, and try to optimize your textures as much as possible.

2. Don't use high-poly meshes. Should be self explanatory. More polygons = more time to render them all. This includes having a multitude of low-poly objects being rendered all at once, which would violate both this rule of thumb, as well as the previous one.