Hi,
A cron plugin have a frequency (period) option in order to give a call frequency and not call plugins too often.
The plugin does not make any difference between main products and variants.
The quantity has to be lesser than the stock limited configured.
Please note that's a less and not equal. So if you set "0", you won't ever be notified because the product quantity can't be lesser than 0. At this moment you have to put "1".
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.