Is there any issue with adding brand new expansion tanks to an existing setup with a couple 1 year old expansion tanks? Or, is it best that all tanks be the same age? Thanks.
Its most ideal to only connect new batteries together. This insures they age together and have as similar of internal resistance as possible.
Using mixed age batteries in a parallel or series circuit can work but the older batteries may get hot and burst if resistance difference is to great which is why its not generally recommended. One way to mitigate the risk is to actively monitor the battery temperatures and disable charge/discharge if one of them gets above a limit. You will have to do some research to find what those limits are and 3rd party products that can monitor and disable power flow.