Silver jewelry can be polished with any commercial polishing cloth. I don't recommend liquid cleaners as this will remove the detail oxidation.
I also wash the pieces in Dr. Bronner’s soap and use a soft very toothbrush to clean the stones. Especially the backs.
I'm in New York which might be the tarnishing capital of the US. I've found if I keep the jewelry in a plastic bag in its black pouch and place it in an air tight plastic container I don't have to polish it so much. It also helps to wear the silver, it seems. There is also anti-tarnish cloth, which can be expensive.
For the pieces with multiple stones I would say be careful. Like take off the rings when you wash dishes. Keep your jewelry away from clorox or chlorine, especially silver. It affect the alloys in the metals. Don't use paper towels to clean as they scratch the metal. Use a very soft cloth or use a toothbrush very gently in the crevices.
Any rings with multiple stones in them are one of a kind and can't be resized. The types and amount of stones have usually dictated their size.
Most of my rings have a lot of detail so resizing is a custom order. I'm happy to do it, but there will be an extra cost and it will take time. Please allow 4-6 weeks.
If the resizing is more than 1 size larger or smaller, it will be a complete reworking of the ring and chances are it will look quite different.