1) Up to your max leadership, and only if you have a free spot in your army. For example: you have 5000 leadership. 1 green dragon needs 1900 leadership, so you can have 2.
2) Yes. Any lvl 5 unit.
3) Use them to modify what Call Collosus will give you. Save the game and use it. If the troop doesn't meet your criteria, reload, use 1 egg or something and use the scroll again. The output will be different. You can use this trick as many times you wish.
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