You can adjust the thrust and amount of fuel in solid rocket boosters which will both have an effect on how long the solid rocket boosters will fire for, so you could tune solid rocket boosters to always run out of fuel first.
I've no idea what button to use on PC, but you can bring up the info dialog for the decouplers from the staging (I put cursor over decoupler stage and press square on PS4), and the info dialog has a decouple button which you can use to decouple it outside of the order of the staging. You don't necessarily have to move them around in staging. You can do that with anything in the staging (fairings, parachutes, etc.).
While above is good for single item, if you require two or more things done at exactly the same time then you need to look at action groups as mentioned.