I was trying the game out with a solitaire game yesterday (playing Apollo) and this card came out fairly quickly with another basestar already there and I thought "...surely not...". I got to thinking: "What if there already are 2 Basestars? Then this setup can't be done, so it must be replacement instead of addition." Guess I was wrong.
Cylon units from crisis cards are added up to the limit of the models provided. For instance, if you were instructed to place two basestars by the card and one was already in play, you would end up with two total basestars. The current player decides which ships are not placed.
Tokens and plastic ships are limited to the quantity provided, and can run out during the game. The current player always decides the order in which a component type is placed, and if there are not enough, he decides which ones are not placed.
Example: A Cylon attack Crisis Card is drawn that requires two basestars to be placed on the game board. There is already one basestar on the game board, so there is only one more that can be placed. The current player decides which of the two basestars listed on the Cylon attack card to place on the game board.