For fire prevention, in the ambient temperature of a house, with wire rating for a house like THHN, you can run the 12awg at 20A, and 14awg at 15A. This is because the wire temp is 90C and you have wire with insulation rated for 90C. The fire rating depends heavily on the wire insulation you are using, and the wire that folks are likely to find here here for automotive use is probably not the same insulation categories. So when determining the max current carrying capability, you don't want generic rules, you want to look at your insulation rating and make sure the wire stays under that. For example, in your kitchen stove you provbably have some wire that's around 18awg and running close to 20A, see this for reference.
http://www.mcmaster.com/#8209k11/=q6rsev note it's 20awg nickle plated copper rated for 840F a is UL listed to support 23A at 89F. Silicon jacketed wire typically has a much higher temperature rating and much higher voltage rating. You really need to look at the insulation when determining if a wire is safe.
Also beware the mechanical strength of the wire insulation. The NFPA70 specifies both temperature ratings and physical stress tests of the wire at the same time, while many UL standards only specify either temperature or mechanical strength. So if you have a hot wire, that's under some stress, that may not be specified by the UL standards.
In our case, you really need to consider what's being wired and what's acceptable electrically. From this page
http://www.powerstream.com/Wire_Size.htm we see that 10A on 8ft run of 12awg will have a voltage drop of .261V. Bouncing your GND around by this much as you turn on and off injectors or ignition devices can be a problem for the analog circuits. Especially if you have a shared GND for that sensor. This is why I have a tendency to run the GND wire back to the ECU harness. Most MCU's are ratiometric devices, which means they are a proportional indicator of your + rail and - rail. If you power your sensor with that same rail, you can often allow the GND or + rail to wonder a bit with out seeing it in the sensors digital numbers after that ADC. However with some wire length, and with some capacitance and inductance, you can see variations caused by various signals delays. So you want to avoid fast transients, but slow wondering is generally OK. The rising edge and falling edge of the injector pulse would classify as a fast transient.
So for a injector, the + lead at 18awg and 1A is fine for both fire and electrical parameters. However the GND wire at 1A would be questionable as it would induce .026V of ripple into the ECU's analog signals.
About a suggested approach, I would suggest spinning small PCB with both connectors on it, with screw terminals. You would start with several short jumper wires under the screw terminals, then you could add change and remove them as requires. I don't like the crimps or the flat spade faston approach as those crimps can be a hidden source of issues. You really need proper crimp tools and proper practices to ensure a good connection, With out those controls, you often induce mOHM's of resistant in places that can cause problems. A couple mOHM's might not sound like much, but You can get into tank circuit issues with stepper motor used for IAC or similar. It's common that the mOHM's in the connector will allow a 12V driven stepper to exhibit more than 300V internal to the stepper motor, which usually results in fire. If you connect a scope at the stepper driver, you'll only see 12V as you would expect, but if you put a sensing wire internal to the stepper motor, you'll see the 300V. If you use a screw terminal, it generally reduces the potential for these mOHM issues. As well it's very flexible and doesn't require special crimp tools or anything like that.