Regarding the fiscal periods, our recommendations is to keep those to minimum to avoid performance slowdown with slices, Data Warehouse and even UI.
You can safely deactivate the old fiscal periods, the data is kept on them and can be activated back. Once you deactivate, just confirm in Administration - Time Slices that the fiscal DWH time slices start dates are not back in time. If so, correct them
Run Time Slicing and Data Warehouse job.
The impact of deactivating old fiscal time periods is that you will not be able to see some of the old data in reporting. So we recommend to keep to 2-3 years maximum in the past in case you need it for reporting purposes.