Hello,
I have a Synology 1621+ with 6 6TB HDD drives running SHR with 2 drive fault tolerance. It’s my main storage for our 3 home computers storing our Word, Excel, etc files. In addition, In addition I use it for my 250K plus large RAW file storage for my photography running Lightroom and Photoshop. I was thinking of adding the M.2 2280 NVMe SSD for cache to try and improve the overall performance of my system. Synology offers two versions, the 400GB and 800GB. My question is could I install just one module for now and install a second later, if needed? I was thinking of installing a single 800GB module to start. A second question is, and I know this is a question that has been asked before in other forums, should I stick with Synology, or could I use a module from another manufacture given the huge price difference? Thank you for any advice you can offer. 
So I would honestly just go for a single 400 gig stick! This is really all you are going to need!
Thanks Will for your speedy reply! Much appreciated. 
Keith