I think you are very wise to wait a little before adding more. Keep testing for nitrites, as it could also be that you are starting the second phase of the cycle (where the ammonia disappears and the nitrites then go up). But if the ammonia disappears and the nitrites stay at 0 for a few weeks, then you probably are cycled.
Guppies are lovely fish, some of my absolute favourites! The only thing I would say is, watch those black widows as I have heard that they can be nippy; of course, guppies have lovely long flowy tails which can present a bit of a temptation to any fin-nippers. But if you set up that old hex, you could always transfer a fish in an emergency (and so MTS claims yet another victim...

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Guppies are livebearers, so they do not lay eggs, but give birth to fully formed free-swimming fry. It is impossible to predict whether any guppy will or will not eat their own fry; some of them do, some of them don't, and the same ones don't always behave in the same way. Unpredictable thing, nature...
Guppies are not quite as hardy as they were, so look carefully at the fish you are buying. Though your seller sounds a reliable person, which is good.
And we do have a special livebearer forum, where the real experts will answer any questions.