In answer to your specific question there are instrument groups which are generally used together in different genres of digital music .
For Classical music which you are addressing:
Orchestra: flute, violin, viola, cello, contrabass, oboe, horns, clarinet, trumpet, tuba, string pad, woodwind pad , brass pad, Timpani.
String Ensemble: Violin, cello, viola, contrabass, string pad.
To get a beautiful sound you must consider much more than what instruments to use but also the role of each in various parts of the arrangement and the mixing of the instruments and how to play them.
Mixing is the stage where you create depth and quality to an arrangement. It also usually where you will add special effects, decide on the stereo placement of voices, adjust velocity and reverb or even decide on the length of a part in the mix. All which create a sound field
There is a 3 dimensional arrangement between panning, velocity and reverb. Panning sets the position on the stage from the left to right, velocity and reverb controls how far back or forward the instrument is located. Pitch can also have a bearing so it is wise to separate similar pitched instruments in different horizontal locations to avoid interference. Panning is critical to the makeup of your stereo image. A stereo image has two basic perspectives, left to right and front to back. Pan midi control 10 control the left and right axis. Volume control 7, reverb control, delay, filtering and ambience create the front and back.
Instruments that we perceive to be closer are louder and are drier (less reverb), because you experience less reflected, sound and brighter. The elements of the mix that are important are up front and we hear them most clearly. Those in the back may have more early reflections infused into the main sound of the instrument and are lower in velocity sometimes the highs in these further back sounds can be rolled to increase the effect of distance. In your mix, you might create a reverb just for these early reflections that is separate from the main, hall reverb. Why is that? Consider being at a concert hall. The loud elements may bounce off the back wall and ceiling even though they are up front. Yet the softer instruments in the back may be imbued with reflections but very little of the sound energy may actually bounce off the back wall.
The main thing here is to keep instruments out of the way of each other so the listener can hear them clearly.
There are also some playing techniques such as: Yamaha demonstrators frequently use the sustain pedal to add ambiance to the sound of strings when playing. Do not use this on low sections because it can sound muddy. Also, make sure you play wood instruments allowing time for the player to take a breath.
There are a couple of articles on the main forum on arranging:
https://psrtutorial.com/lessons/workshops/H5-arranging.htmlAs has been said by others you need to do a lot of listening to what others have done and take a look at some of the styles. Some pieces can best be created using mutli-tracking,
My two cents.
Ed B