A trombone is an interesting instrument. Brass instruments by definition are a loud bunch. The trombone is tricky to play because there are no valves to press to get a certain sound. The trombonist must know just where the slide must be pulled to make the tubing the proper length for any given note. Like a singer, the trombonist has to be able to listen and make sound at the same time. Brass instruments, however, cannot be drowned out but voices can.
When we sang Banks of Doon at Embro Ambassador Evening, we had Heather and Leslie Galloway who played the violin obligato for us. It was a wonderful addition to the sound. Anything louder would have spoiled the overall feeling.
Recording your practices really helps you to hear the blend. If you have lots of microphones and mixing board and talented technicians, then you can do lots of things and even 76 trombones would work. For most of us, who use simpler equipment, a piano is usual.
I think we will stick with our amazing Kristy and the keyboard with occasional additions like guitar and violin. Having a truly good accompanist is a blessing not to be ignored. You can have all the wonderful equipment in the world but if your accompaniment is played poorly, it spoils the sound. Period.
The bottom line is that you must choose what works best for your singers and your sound.