Can Soft Arranger work on its own?
Yes, it will “work” on its own but since it only produces MIDI data you won’t hear anything. You need to hook up Soft Arranger to multiple synthesizers or sound modules to hear music.
If you set up everything as suggested with Reaper and the TX16Wx vst plugin you will have a system similar to a hardware MIDI arranger keyboard.
You can also hook up Soft Arranger with your existing DAW or hardware synths and you will get music but the styles won’t sound exactly as intended. For this type of setup you would need to create your own styles.