I guess it would depend on the "competitiveness" of the group in general. But my thoughts are that a Knockout, Strikes, or general Match play where the guests are eliminated as you work your way towards an eventual champion is not the right format for a teenage girls PARTY. I mean, what teenage girl wants to be knocked out of the competition and have to sit by and either watch the winners play or go find something else to do. This sounds like a great format for a serious competition... but not for a PARTY.
In the spirit of having fun at the party, I would recommend that you just setup a rotation where everyone plays each game 3 or 4 or how ever many times you have time for. Then have them record their score on a master log for each game. At the end, rank each player with a 1st, 2nd, 3rd, etc based on their best score (once per game). The guest with the lowest number after adding up all the scores would be the winner. Lowest possible score would be a 4 (for 4 pinball machines) and highest would be a 80 if you had 20 guests. <full disclosure... saw this in another topic-post by Nveld>
This way, everyone is playing til the very end and even though the results may be known based on who is doing well, there is still a little suspense in the "reveal" of the results. Keeps everyone involved and can still foster a little competitive spirit.
Good luck... sounds like fun!!