Hello, yes that is possible. I had a DX513 extension connected to my DS1515+. The DX513 is only available second-hand, but the DX517 is still available. Depending on the model combination, the extension can be integrated into an existing volume or must be set up as a separate volume (see info at Synology itself).
About APC I can't answer the question, but I first had the APC by Eaton 5SC500I, now the Powerwalker VI 750 R1U, both work fine and are recognised by the Synology DS1515+.
Hello,
The Compatibility List on Synology is always up to date and shows which disks work. https://www.synology.com/de-de...
In a SHR raid, the number of disks -1 is always taken as the effective storage space.
10TB disk has an effective usable capacity of 9.1TB. So 4 x 9.1 = 36.4TB.