No restrictions on reserves. However: If at the end of any game turn (eg after player 1 and player 2's turn one)One player has no models on the board the other player wins automatically. So unless you have reserves that can arrive on turn 1, you'd lose after your first turn!
For those grey knights, you would have to take them as the special nemesis strike force detachment (p94
GK codex) which lets you roll for reserves on turn one and hope you don't roll all ones and twos.
Unbound: take what you like, it doesn't matter. One exception: the warlord must be from the primary detachment. In unbound, its not an issue, but some missions, campaigns or scenarios may have rules that effect primary detachments, so you still technically have to choose. Whatever faction the warlord is from, all other units with the same faction in the unbound army form the primary detachment.