Hi,
Tricky
, very tricky depending on the content.
first: retina vs non retina. I usually do everything retina, and then older device will simply by default scale everything by two. SOOOO everything has to be divideable by two, else you will have half pixels artifacts and all.
different screen ratios. Use screen anchoring, no absolute positioning, that's the only way.
both of the above can be done withotu any third party 2d plugins.... but really, if you have a lot of ui elements, I would strongly recommand you go for 2d toolkit ( which supports multiple atlases for each resolution), or NGUI ( which has some additional plugins to have different atlases as well).
Also, depending on your content you will face major issues with camera field of view, and some camera settings will have to be different. so likely you won't be able to build an universal IOS app without tweaking values at least for the camera.
The other solution is to build your ui as separate scene that you load additively, each IOS device having its own UI scene. that's also a very good approach and a good way to create some very powerful frameworks.
bye,
Jean