Daisy is right there - if cycling is complete and you did it to a good level then there is no reason they can't go in at the same time

But as she says, if it was only a small amount of ammonia then best to play it safe and add one then the other - I'm afraid I can't remember what level you cycled to - can you remind us?
On the food thing - sinking pellets are great for fancies but not just the one pellet per day! Feed lightly to begin with and then you'll need to up the rations while keeping an eye on water quality
How big are these fancies? And do you know how old? Size and age will affect, to a certain degree, how much you feed them so a little more info would be great