its need small engine and in sfs there is only one small engine that is ion ant it is in DLC. so it we need to make a space for korbial engine so it can be a little hard
No, it really, really
really isn't hard. In fact, it's easier than it was before because engines can draw from the side of tanks, so you don't even need to rotate a fuel tank to feed it. You just stick an engine to the side of the booster and voila.
I'm curious though. You don't have the DLC. So the boosters you're going to be using are going to be tiny. I don't get why you even need separation motors installed.
Even a single Kolibri engine is going to be too powerful, unless each individual booster is going to be in excess of 200tons.
I know what you are trying to do, a Korolev Cross on a base game Soyuz. Which is a neat idea, except this isn't KSP and the small motors required to do one properly with a booster set that small just isn't gonna work.