No one can answer without knowing which model system you have.

It is true that the 3-cell battery allows for a carrier and 2.5" drive to be fitted. It's also true that you're going to lose about 50% of your battery runtime in downgrading the battery -- and if the system has a high resolution (UHD) screen, don't even consider the downgrade; you won't get enough runtime from the smaller battery.
You will need to purchase a bracket, cable and screws to mount a 2.5" drive, so when ordering the battery if you plan to install a 2.5" drive, remember to order those from Dell or a Dell parts seller.

The part number below is the correct one for your system -- that said, finding new OEM batteries for this model may not be easy, due to its age.
https://www.parts-people.com/index.php?action=item&id=29927

Looking for suggestions on a site, brand, etc., for the battery replacement, as I've found out the hard way, cheap isn't always the best option!

Also, I'd like a stand with fans for this, as well.

XPS 15 9560