Upgrading my iMac 21" mid 2011?
Upgrades
I'll address your upgrades in the order you presented....

The iMacs from 2011 use the Intel Core i5 or i7 Sandy Bridge processors which support a maximum speed of 1333 Mhz.  (iMacs don't use the Core i7 Extreme which support 1600 MHz).    However, your particular iMac will support up to 32GB of RAM1.  My suggestion is to max out your memory.  I have used this exact 16GB kit from Crucial to upgrade my personal iMac (you will need two 16GB kits total).

Swapping out the SuperDrive for a optical bay drive caddy and making a fusion drive2 is an excellent idea.  I have done this exact thing in MacBook Pros with excellent results.  However, in the iMac, you will find a 3.5&quot; hard drive which I would swap out for an SSD and then install a laptop hard drive instead of the SSD in the optical bay caddy.


If you haven't changed out that hard drive, keep in mind, it's going on 6 years old now which means it near the end of it's useful life.  You are taking the time and effort to do this upgrade, now is an excellent time to address the drive before it fails on you.  What you can do is get a USB drive enclosure and turn it into a Time Machine back up drive.
If you need step-by-step directions on how to do this, ifixit.com has an excellent guide.
Make a Time Machine Backup
Before doing any of this, do a Time Machine backup so that you have something to fall back on (and migrate your data from).
Do a clean macOS install.
Do a fresh install, don't restore from Time Machine.  Instead, use the Migration Assistant.  Doing a clean install will ensure that orphaned files and apps you no longer need is no longer &quot;floating&quot; around your system.
Before beginning your upgrade, it's a good idea to take inventory of the Apps you have and want and any custom settings.  Doing a clean install is like getting a new Mac and an excellent opportunity to evaluate your workflows and preferences.

1 EveryMac.com:  Official Max 16GB, 3rd parties have verified that it will support 32GB of RAM
2 Macworld:  How to make your own Fusion Drive