Good Method to Add Wiring For Add-Ons

So here’s what I’m thinking on adding circuits. I have this: Summit Racing SUM-900356 Summit Racing™ Fuse Blocks | Summit Racing
It’s for:
-tach
-oil pressure light
-volt gauge
-electric ATF temp gauge
-fuel pump (Holley blue)
-line lock
Research tells me that the last two draw 5 amps each, and the first four draw very little. I’m thinking 15 or 20 amps at the most, total, Probably 15. It need to be switched 12v.
I’m going to use a relay because from searching on here, I see that switched 12v is hard to come by. (74 low option Dart Sport). Do I tap into the blue field wire that also goes to the ignition, voltage regulator and ballast resistor? (to feed the relay 86 terminal)
I’ll fuse the 30 terminal, the one from the factory starter relay.
Here are my questions, as I’m not good with electricity:
Is this a good choice for a relay? Tyco / Bosch 30/20 Amp Relay – CE Auto Electric Supply
I have no idea how to select one. Amp rating? What does 30/20 mean, as an example. Oh wait, continuous open and continuous closed? Still, that confuses me. Specifically, what will I need? I know it needs to be NO (normally open).
Does it matter if there are two 87 terminals? Does that just leave an option on the table for another use?
@Mattax @67Dart273