Hi Anssi!
You can choose any amount of players with the all preset. It's just not possible to switch to a distinct player as a soloist. And I'm still not sure why you would want to do that. With the Dimension Brass there were 4 players. With the Dimension Violins we have 8 players. So there would be far more possibilities to make combinations, but there is only a limited number of keys for matrix keyswitches (the lowest octavce). In order to not get the Sibelius sound set out of proportion the keyswitches have to stay the same for all Dimension Strings presets (Violins, Violas, Celli, Bassi).
We have chosen Player 1 on purpose to be Player 1. If you prefer Player 5 (or any other player) as a soloist, you can replace the Player 1 matrix in the all preset with a Player 5 matrix. To do so just load the Player 5 preset, save the matrix, load the All players preset and substitute the Player 1 matrix.
I haven't seen a score yet, where we have a string group change to different soloists within this group and every soloist is notated on the same staff. If you want to make this kind of music you can still notate it on more than one staff. I can't see why this should be limiting.
Best,
Andi