Hi Fernando,
My understanding of data ageing is that for SAP-delivered tables, you should really only use the SAP-delivered data ageing objects. The ability to create your own data ageing objects is so that you can include your own (Z) tables in your data ageing strategy. Although it is possible to create data ageing objects for SAP-delivered tables, I have a feeling that it might not be such a good idea - at the very least, you may run into problems as you said when SAP does deliver objects for those tables.
To optimise your SoH memory usage, you might be better off looking at HANA Dynamic Tiering and NLS using the Data Lifecycle Manager.
Cheers,
Kevin