Several modifications planned... what order should I do them in?

For new MBPs optical bay for the SSD is just as good as the original HDD bay. Your plan seems very sensible to me, you can install the RAM and SSD at the same time while the computer is open, most likely it will be fine, if there is any problems you'd just have to double check if either of them is installed incorrectly but it should not be difficult either way.

 

The order doesn't really matter. I'd restart after each upgrade, to make sure all is working properly.

 

Since you didn't mention it in your post, I have to ask if you bought a hard drive caddy. If you haven't, you'll need one.

 

I'm in the same position as you, mid 2012 13". I've just put my Samsung 830 128gb in an don't think I'm bothering with ram for a bit now, it's lightning fast with 4gb.

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Also I installed the os on the ssd using a sata-USB cable before I even took the laptop apart and I've had just the ssd in since that came before the SuperDrive caddy. Caddy arrived today so I'll be putting the hdd back in its original place because that looks to have better protection against vibration

 

i had my ssd in the optical bay and hdd in original bay earlier, since then its stopped working and ive had to swap them. boot time seems faster with the ssd in the proper hdd bay

 

In the newest crop of MBPs there is no difference, in fact there may be a slight benefit to keep the HDD in its place for better shock protection, which the SSD does not need.