What they told you at the garden center is correct. Pups should be at least 1/3 the size of the mother plant, and for more difficult species the bigger the better. You don't say what kind you have, so it's hard to tell from the size how close to mature your pups were, but by your own description I'd say that was too soon. Maybe you could glue them back on and see if they'll grow some more! Okay, no, that won't work. There's a chance they may make it, depending on what type it is, but even a tough species (like some of the Aechmeas, for example) will grow very slowly if it's not mature enough. If it was a Guzmania or something a bit more touchy like that, then kiss them goodbye. :-( If the mother is still alive you should be able to get at least one more out of her, though.

Your pups are quilling. This is a common problem with many broms, particularly when the cups get too dry, but in other cases the tendency to do this can be genetic. The best thing to do is to gently wiggle the leaves back and forth as soon as you see it starting to do that. You may have to repeat this several times as it grows out, or shoot some water (again, gently) between the leaves. If the base of the leaf is completely wrapped around itself you may have to slide something down there to pry it open, but too much force will snap the leaves, as they tend to be brittle when they're doing this.

I don't usually cut my pups off preferring to pull them off. Put your thumb between the plants and slowly pull down. By cutting and trying not to damage the mum, you may not have cut in far enough to get the heel. Without this most will die. Your pup should have its own roots so just pot it up in an open mix with a small amount of fertiliser, water in the cup and water the mix when needed. Put a bit of fertiliser around mums pot and you may be lucky to get more pups. Water in the pups while still on mum is beneficial.

I have mine in pots and leave them together as much as possible. With these Aechmea faciata the mother plant dies after flowering anyway, so stress on it is a non-factor. You could, however, argue about the jumbled mess. Grown in the ground would be less messy. These are two pots. tj