Welcome to the world of handloading! You've posed some good questions. Handloading can be a VERY rewarding hobby. I've been doing it now for a VERY long time. One of the MAIN goals of handloading is to learn to do it SAFELY! Remember, a loaded round is basically a bomb. As long as I've been doing it, I still catch myself every now and then making a small mistake, usually due to a lapse in attention or a change in my technique. Every experienced handloader can tell you of a handful of "near misses" he's had.
The single best piece of advice was given a few pages back before the "distractions."
I think you need to buy some reloading manuals and spend some time studying before beginning to reload.
The BEST thing you can do right now is to buy yourself at least one (2 would be better) reputable handloading manuals. This will provide you basic, and safe, handloading practices. I've found the manuals from Hodgdon, Hornady, Sierra and Nosler the most useful. You'll also get safe, well-tested load data (think of them as recipes that tell you what powder to use and how much powder to use w/ certain bullets) . You have to beware of handloading data off the 'net that's NOT been pressure tested. Hodgdon also have a good online site w/ loading data.
Now, to answer your original questions:
1. There are some situations where some seasoned handloader might substitute small rifle powders for small pistol powders. I would not suggest this for you since they're not really interchangeable.
2. There are only a few instances where a single powder could be used for both the 223 Rem and the 9mm. I don't think you'll enjoy the limitations you'd have to live w/ by using a single powder for both. Get different powders for the two. For the 223 Rem, I like Hodgdon 322, Benchmark, Vihtavuori 133 and Vihtavuori 135, but there are a LOT of great 223 powders.