Dell 5580 add hard drive when SSD installed ?

It's either M.2 or 2.5" - not both. There two drives occupy the same physical space - with a hard drive installed, there's no way to secure an M.2 drive.

what is the m2 cache drive and how does that fit into a configuration ?

These come with m.2 2280 SSDs - not the older mSATA cache drives.  The M.2 2280  requires a mounting bracket that will block the installation of a 2.5" drive (and if there's a 2.5" drive mounted, there's no place for an M.2 2280 drive.

mSATA cache drives were common on older systems -- with the advent of hybrid drives with built-in SSD caching (and the availability of much faster M.2 SATA and NVMe drives) they're obsolete technology now.

thanks for the insight. Does the m.2 2280 come in larger capacities and is there a limit to capacity size on the 5580 ?

You can get 2280 drives up to 2T in capacity now -- though the prices are high.