I am wanting to replace a shower fixture. My question, can I replace the valves and handles and not the manifold in the wall. If so how do I know the valves will work w/ the old manifold?
You can try to purchase new valves and handles -- they may be available for newer fixtures from major manufacturers (Delta, etc.). I struck out looking for a replacement for my 20 year old fixture earlier this year. They will need to be from the same manufacturer. I would bring your old valve into the store to compare it to the new valve. Home improvement stores carry some replacement parts or you can try to special order from a plumbing supply warehouse.
Make sure the thread patterns are same as metric and imperial are different. If you can find out what make and model the shower valve is I would recommend contacting them for newer options they may have on the market.