I laid out the board already, so hopefully I will not have to make too many changes - this is what I have now:
Input power 12V goes to a 7805 regulator. At the regulator, I have a .33uF ceramic capacitor at Vin and a .1 uF ceramic at Vout.
I have three main indicator lamps driven directly from the 12 volt supply, of which only one will be on at a time. They take about 210 mA at 12V, so next to the connectors and drive transistors for those lamps, I put a .33 uF ceramic and a 10 uF electrolytic capacitor. These lamps will not be switched at high frequency (only << 1Hz).
Vout from the regulator I assigned the net name VDD, and it powers the PIC, which supplies the base drive current to all the switching transistors. At the PIC, I have a .33 uF ceramic and a 4.7 uF tantalum capacitor between VDD and ground. (I used 4.7 uF tantalum because I remembered I had some in stock from another project, but later I remembered the ones I have are rated for 100 V and quite expensive. Therefore, I am going to reorder different ones anyway, so the size/type can change if needed.)
For powering the segments of the multiplexed 7-segment LED display, and my other LED indicators, I'll use a 74AC573, which can drive up to 24 mA per output (I'll have to adjust my resistors so I don't exceed the current rating). I was going to use a darlington array as suggested, but it seemed that they invert the signal, and I couldn't figure out how to hook up the "freewheeling diodes common" terminal. For powering the 74AC573, I created a separate power net called VCC, connected to VDD through an inductor. I didn't know what size inductor to use. Do I have to worry about resonant frequencies with the inductor and capacitors in the circuit? I chose 22 uH because those seemed common on Mouser, not because I thought that value any better than anything else. On the VCC side, I used the same sizes of decoupling capacitors at the '573 that I did at the PIC. If resonance could be a problem, maybe using the same capacitors on both sides of the inductor was also a bad choice.
There is just one ground plane, and I made it very solid and well-interconnected.