if you have the necessary cards (fusion materials) in your hand to fusion summon yes, since fusion monsters have their own deck area.
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A Fusion Monster can never be Normal Summoned, as it is impossible to put them in your hand. A Fusion Monster must be Special Summoned, be it Fusion Summon through Polymerization or similar, or a regular Special Summon by any card that can do it.
A Fusion Summon is a Special Summon and things that can be used against Special Summons can also be used against a Fusion Summon. But not all Special Summons are Fusion Summons, even if they do so happen to summon a Fusion Monster. For example, Cyber Stein Special Summons a fusion monster, it does not Fusion Summon it.
The situation that you are describing, in the first place, cannot exist (a Fusion monster being in your hand). If a Fusion monster on the field is supposed to be "sent to the top of the owner's deck" or "returned to the owner's hand" by an effect it is instead returned to the Extra/ Fusion deck.
Cyberdark Dragon cannot be summoned except by Fusion Summon. Cyber Stein does not 'fusion summon' the fusion monsters is summons, it is only a special summon. It therefore cannot summon monsters like Dark Paladin, or Cyberdark Dragon.
No, cards like 'Polymerization', 'Future Fusion', and 'Fusion Gate' cannot be used to summon any of the XYZ Fusion Monsters. These Fusion Monsters cannot be Fusion Summoned and requires you to banish the monsters listed on the Fusion Monster from your side of the field in order to Special Summon them.