Chain length can be adjusted when you fit it but the width has to be correct. 7 and 8-speed chains are the same width but 9-speed ones are narrower, with 10-speed narrower again. Choose the right one to match your gear system. You can use a SRAM (Sachs/Sedis) chain with a Shimano gear system - in fact, most people do! When you join a Shimano chain, you must use a new replacement connecting pin. If you change your chain, you must replace the cassette sprockets at the same time or the new chain will skip over them under load.