1) No, it's not only for Intellijel tiles. There are other companies (Plum Audio comes to mind) that do modules in Intellijel's 1U format. You just have to be careful when checking listings and, also, pretty much all of Pulplogic's tiles are out of the question.
2) It's not that modules are "efficient". Fact is, a modular synth is about the most INefficient musical instrument there is. Most of what's inside the case is air, every connection has to be handpatched as a rule, patches require constant adjustment, and nothing works as you'd expect if you happen to look at your rig wrong. If you want "efficient", you're looking at the wrong thing altogether! That being said, the Intellijel tiles are USEFUL (this is the word I think you're looking for) in that they can replace 3U module functions, which then opens up more room in the 3U rows.
3) Garfield is correct: if you're not comfortable with a 1U row, get a different case. That being said, tiles are very useful, especially for utility functions, and the panoply of original format tiles shows this. But given that 1U tiles are just shrinky synth modules, it gives me some pause that if you're having trouble sorting out the usefulness of those 1U modules, there's quite probably some "gaps" in overall synth knowledge here.
4) DO NOT jump into this feet-first at top speed, with full money in evidence. Judging from the questions you've posed above, it could well be that you'd be better off learning the basics of this with a patchable synth, not a full-on modular. That way, you can sort out how this all works and have a device that can form the core of a larger system later on, once you've gotten more chops and more comprehension about the subject. Mind you, you can get well into modular turf with a patchable; something like Pittsburgh's Voltage Lab might make a useful start here, and if you were to pair that with something like a Plankton Ants! or Make Noise's 0-Coast, then you'd have a fairly powerful system that would allow you to work out where you want to go with this without spending the massive pile of cash that a full modular would entail.
Remember: modular synthesizers are really neat and cool looking and all that...but they have the potential to be the most hideous money sink you've ever encountered if you go into this without the requisite knowledge. Not everyone needs one, either. Really, they're best in the hands of musicians and sound designers who've exhausted all conventional sonic possibilities and who now need to go "off the map"; they're not an essential, despite what loads of YouTube videos might lead one to think.