Understanding transformers and power supplies... a few questions.

So I get the basic gist of what a transformer with dual primaries and dual secondaries or a single primary and dual secondaries is. I have about the same understanding of what windings are in relation to the secondaries. I also understand that there is the concept of running the secondaries in parallel or in series or if you wanted to create a center tap. As I stated my understanding is quite basic. What I'm trying to wrap my head around is how to figure out what transformer to buy if you needed to give enough juice to a power supply. In my current situation that would be a bi-polar power supply. Do you get a transformer with two secondaries that are rated for higher than what your supply is regulating ie two 20VAC secondaries for a +/-15v supply. Or does that not matter?

Also, is there such thing as a transformer that has dual secondaries that are at two different voltages? ie one secondary at 5VAC and the other at 15VAC. If so, what is it called and where can you get one?

My other question has to do with power supplies. Let's say I build this bi-polar power supply and it can supply 100mA per each rail. I'm assuming this bi-polar supply can do that...

Sjöström Audio - SPS02 Sjöström Power Supply

If I wanted to power other components other than my main component that needs +/-15v could I do that? ie a component that just requires +15V. I might be answering my own question, but is the limitation based on how much current the component draws? So let's say component A requires 50mA from my bi-polar supply and component B requires 25mA. Then I have 25mA left over from the 100mA that the power supply can supply.

Thanks in advance for your help in helping me wrap my head around this.