How exactly does Xenograft work, and not work?
All creatures already on the battlefield and all creatures that come under your control later are affected by Xenograft. No if's and but's. As pointed out, it's a static ability that generates a continuous effect. If Xenograft would affect only the creatures that are in play at the time of its casting, the wording would be quite different and would probably involve counters to mark the affected creatures.
Creature cards not on the battlefield are simply that: Creature cards on the stack, in your hand, library, graveyard, command zone, exile, ante, or out of game are not affected by Xenograft.
Creature spells are not creatures, but objects on the stack. You control the spell, but a spell is not a creature, so Xenograft does not apply to them either. Only if and when the spell resolves does it become a creature to which Xenograft can apply.
Creature spells are spells, not creatures:

111 Spells
111.1. A spell is a card on the stack. As the first step of being cast (see rule 601, &quot;Casting Spells&quot;), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner's hand. (See rule 405, &quot;Stack.&quot;) A spell remains on the stack as a spell until it resolves (see rule 608, &quot;Resolving Spells and Abilities&quot;), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, &quot;Spells, Abilities, and Effects.&quot;
302 Creatures
302.1. A player who has priority may cast a creature card from his or her hand during a main phase of his or her turn when the stack is empty. Casting a creature as a spell uses the stack. (See rule 601, &quot;Casting Spells.&quot;)
302.2. When a creature spell resolves, its controller puts it onto the battlefield under his or her control.

Xenograft only affects creatures on the battlefield:

603.6b Continuous effects that modify characteristics of a permanent do so the moment the permanent is on the battlefield (and not before then). The permanent is never on the battlefield with its unmodified characteristics. Continuous effects don't apply before the permanent is on the battlefield, however (see rule 603.6e).

user1873's answer also covers this well.