If I were to generate tiny pulses of electricity and feed them directly into my interface (for the sake of some rhythmic analysis I'm doing), what wattage (volt/amps) should I limit them to to avoid damaging anything?
The definitive answer will be in the technical specification for your audio interface.
If your interface has separate "line" and "microphone" inputs, use the "line". If there is only one input, it will probably handle "line level" signals without causing any damage.
"Professional" and "consumer" audio equipment are designed to different standards for line level input, and professional gear is usually much more tolerant of gross overloading without causing any damage. Of course overloading will cause severe distortion, but that may not matter if you are just recording pulses and using them as timing information.
As a rule of thumb, a peak level of 0.5 volts should work fine with any type of interface, i.e. the signal level will be high enough to get a good recording level but without overloading anything.
The line input impedance is usually high (several thousand Ohms) so the "wattage" you need to supply is too small to be an issue - just design your circuit to produce the right voltage level.
Nominal level is 0.775v (consumer Line), 1.2v (pro Line). Some interfaces have input level trims, some require you to optimise level externally. If you're worried about damage, more than this (within reason) will sound nasty but won't actually break anything. Amps aren't your problem. It's a high-impedence input, current drawn will be minimal.