You can get a sprocket checker. The classical one was that by Rohloff but KMC and some other manufacturers have their copies of the tool out. Smaller cogs can indeed wear faster and you can principally replace just some, but it quickly ceases to make financial sense. Different operation for the new and old chains does not sound good and going back to the old chain is not a good solution. I would replace the cog and even chain now and already order a new cassette. When the drivetrain is not being cared for, 1700 miles for the lifetime may be close to normal. Else, with the new cog, you may wait till the 0.75% you cling to for the chain, but it may be a rather short time - the wear progresses at an accelerated rate.