Don't think it's going to be possible to get Rooney, Sandro, Klaassen, Sigurdsson and Giroud (if he comes) into the same side
Rooney, Sandro and Sigurdsson can all play slightly wider, so that helps but Klaassen and Giroud are both definitely central players
Having said that, I don't see why this should be a problem - we want to be playing 50-60 games this season, so there should be plenty of playing time for all of the above. Based on what we saw from Koeman last season, we will alternate between 433, 4231 and occasionally 343 at times this season - we need tactically adaptable players and different options, so when I see people naming their "first XI", I tend to think it's a waste of time because we are striving to not have that. It's different when all you have to worry about if the league, but if we are being really serious about all of the competitions we're in this season (which would be for the first time in recent history), we won't need to try and shoehorn every attacking player we have bought into the side