In the reality of things, the simplest and most effective way for your cordless tool to automatically cutout when the draw is too high is by measuring the voltage. So, the way your cordless tool decides what is "too much current" is really, in all likelihood, an indirect judgement based on the series voltage under load (or under stall, as the case may be).
So matching of the discharge and charge rates and capacity is important, but not that important. You might lose some torque if your replacement cells are weaker in C rating. OTOH, if you were to use cells with super high discharge capacility, you might end up with so much torque you could burn out a motor winding or other component by the time the cutout kicked in. But in reality, if you stall or grossly bog down the motor, you probably will let go of the trigger. (If not, you should).
The most important thing to do is to make sure your new cells are not being charged too fast by the charger. If you have one of those superfast chargers, this is a source of danger you have to pay attention to. The other important thing is to make sure each pair of cells drops in voltage at relatively the same rate. Similar to the way the tool decideds what is too much amps, the way it decides that the battery is dead is in all likelihood based on the cumulative series voltage of the cell. Yeah, it would be way way better for the tool to measure each pair of cells and cutout when the weakest one dropped to a certain limit. But most of these tools won't do that. So if the capacity of your replacement cells is way off from the rest, the worst cells will be repeatedly damaged by the time your drill decides to turn itself off. This is why in many cases there is just one really bad pair of cells in a battery, and the rest might be great. It's that one pair that is repeatedly abused.
That said, if your new cells are not greatly matched, you might just decide to stop using the drill when the battery indicator gets to 1 bar or something. Don't keep using it until it dies. Put the battery on the charger preemptively. As long as the charger is not crap, it is fine to charge a li ion battery frequently. You don't have to deep cycle it or other voodoo you might have learned with other battery types.
So if you are careful and you pay attention to what you're doing, and you observe the results and make sure the cells are staying within their limits when on the charger, you can probably get some useful life out of it. If it's not worth your time to take some voltage and temperature measurements/obserations for the first few cycles until you're sure everything is fine, then either don't do this yourself, or maybe put your charger in the middle of your yard when you want to use it.
The charging is the most dangerous part, because no one has the patience to watch the water boil. OTOH, if the tool blows up and catches fire while you're using it, you might get a few scars, but you won't die. And if you are observant of the battery temp, even just touching the case the first few times you use it under heavy load, it is unlikely to ever get to that point.
But you do this yourself, you take your own chances. This applies whether or not some other "expert" endorses your battery choice with their stamp of approval. Better you use a completely inappropriate battery but do the proper experimentation and observation than to get the best cell and think you're fine.
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