You don't have to wait any time at all before adding your plants. While ammonia, nitrites, and nitrates are harmful to fish, your plants actually feed on them.
I would point you to Seachem Prime for a conditioner, but there are many products that will do the exact same thing just as well.
The best way to completely eradicate the snails at this point would be to empty the tank completely and wipe it out with a mild bleach solution (rinse thoroughly). Since you're planning on re-using the gravel, I'd give it all a soak in bleach solution as well (again, rinse thoroughly - note: this will also kill any beneficial bacteria that may still be present). If you want to avoid the bleach, you can try trapping (you'll never get them ALL), or, like you said, loaches, or even assasin snails. To be honest, once you have the tank established, as long as you're not overfeeding, the snails should eventually die out on their own.
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