Being more electronically oriented that flight oriented let me offer a few opinions and maybe a description.
To hook directly to a mag it might be better to connect you rpm meter through a capacitor of .1 to 0.01 microfarads, 600 volt rating vs a direct contact. Same for any motor that is using points. This would deliver the required spike to the tach with out lowering, very much, the charge needed to fire the coil / spark plug. I assume the tach already has it?s own power connection. Find a film capacitor from on line or local electronics store. For the tiny tach, if you are really just winding a few turns around a spark plug or coil wire I hope the insulation is VERY good on both. Any leakage will almost surely over voltage the tach and destroy it. If you do not use a shielded wire with the shield grounded you will also pick up a lot of trash noise from the rest of the ignition system giving unreliable results. Hopefully your sensing wire is not immediately bundled with the rest of the ignition wires. Bundling wires can cause enough cross talk that fools the tach?maybe. To me there should be a ferrite that goes around the wire with a coil around the ferrite for you to attach the tiny tach to. This provides electrical isolation, and signal gain / noise rejection. How many turns and which ferrite is a good question. I do not know what the input voltage the sensor on these tachs can stand but I would have a pair of zener diodes attached back to back across the tach input at their specified voltage to try and protect it.
