To clarify mel's post, Alkalinity at this early stage is still very important, coral or no, because it stabilizes pH, which absolutely has a bearing on fish.
I think the reason mel is saying alkalinity isn't very important yet is because there's nothing in the tank yet that will deplete it. So, assuming you're using a quality salt mix, the alk will take care of itself until you start adding coral. Nevertheless, alkalinity is a critical parameter for fish.
I also notice you have a lot of coralline algae (purple stuff) on your live rock, that stuff eats up magnesium in my experience. Which means your magnesium and therefore calcium levels could be rather out of whack by the time you're ready for coral, so while it may not be important yet, I would definitely have the tools to know what it is BEFORE you add coral. As we all know, calcium is very important for reef-building corals. But what is less appreciated is how the appropriate magnesium levels are necessary in order for that calcium to be able to be used by the coral. I'm hazy on the actual chemistry, I just know that it's difficult to keep your calcium where it should be if you don't also have magnesium where it should be, and in your case magnesium could deplete faster than anticipated due to the amount of coralline algae.
Personally I would go ahead and add a hearty fish now, with or without a clean-up crew. Much of my clean up crew died because my tank was too clean, and it wasn't nearly as clean as yours is. To me it makes more sense to add a CUC after there is something for them to clean up.
As far as food is concerned, variety is important. Most experienced folks seem to offer a variety. I find it really depends on what fish you get. Some food is better for surface feeders, others for bottom feeders, some is better for herbivores (algae eaters), other is better for carnivores. Flake can be a good all-rounder, but it won't take long before you have a fish that is quirky is some way, and more specialization food-wise will be helpful. For example, I have a goby that never ventures near the surface, so if I only fed flake he would starve because the flake floats long enough that it all gets eaten before any of it sinks to his level. Frozen is the next most popular, and there's a lot of variety to be found. Mysis shrimp is a very popular form of frozen food. That's what I use the most, but it's definitely not the only one.
