To keep this general, here are some things to think about:
DAS: Direct Attached Storage - Thunderbolt, eSata, USB 3.0, FW800, FW400, USB are the rough pecking order. FW400 is better than 100 MB ethernet, and FW800/USB 3.0 are equivalent to Gigabit Ethernet.
NAS: The network matters - latency over WiFi can be a big bottleneck as can the controller chip / lack of cache in less expensive NAS.
Think in terms of the bottlenecks - For streaming a song, AirPlay shows you what to expect - a slight delay, but the system can buffer the music to overcome little lags and delays due to network burps. DAS is much more responsive - especially for little file input/output as opposed to streaming a large file or song.
NAS does offer some nice features like cross platform sharing and with a good network can be shared effortlessly with several clients that don't mind sharing the speed of the device. DAS is designed for one user and optimized for speed - especially for database workloads like iPhoto.
NAS can be harder to back up, where DAS works easier with whatever backup program you use on your mac.
Your iPhoto database will be the last thing you want to move. It has no provision for splitting the large files from the database files and the latency of reading / writing to the NAS might make you feel the program is too slow. Try it out, but don't be surprised if it's the least responsive by it's design.
You want programs that can use local database and thumbnails on the fast SSD and store large original files on NAS or slower storage.
Aperture works amazingly well if you find iPhoto too laggy when stored on the NAS.
I would start with the iTunes media folder - move that to the NAS and see how you like the performance.
Then get a tool like WhatSize or DaisyDisk so you can quickly measure what folders on the SSD are largest as you prune things down.
You might find you don't need all the audio loops from Garage Band or other surprising folders that make a difference in what you want to bring along on the SSD.
Have fun with the tweaking - you'll learn how things work. Also, it won't be long until ThunderBolt storage is more available - your NAS might even have a DAS connection for when you really need some speed to access / move files to the NAS.
Don't forget to back up your NAS. CrashPlan might be a great thing to try for that.
There was a time when it "worked" to plug an iPod into separate computers with the same Apple ID. Have you tried just plugging them in? I think the key was that you need to manually manage music and videos on the iOS device. You probably don't want the computer syncing automatically either. There is a setting for that in iTunes preferences.
Syncing iPhones with multiple computers is a big bag of hurt. I don't think that this works at all.
Hopefully iCloud works perfectly and solves all these problems for us sometime this fall.
Best Answer
I've been using SD cards more and more just as you mention and I think you'll be ticked with the benefits of having double the storage.
Here are the tools I use to get the data I need on relative speeds so I can choose what should be stored on the drive and what shouldn't go there.
The Daisy Disk tool is really stellar for quickly seeing which chunks of data are big enough to be worth your time to handle. In my case, the first chunks I will move to the card are:
/Library/Application\ Support/Logicto the cardI also use BlackMagic disk tester to check how the speed of both drives is actually so that I can keep the things I don't care about being slow on the slower SD card (or pay more for a faster SD card).
OS X would run fine with your entire user folder on the SD card, but make sure you have a second admin account on the boot volume if you go that drastic route.
As far as opening files - most applications will run just fine if you use Finder aliases or command line sym links to point to the storage card once you move files off the boot volume.
Also, if you are bugged by HiddenStorage in your setting, download the full version of DaisyDisk - it runs for free if you have bought the app on the MAS and adds two critical features for me: scanning as an administrator and collecting files to then delete them
Be sure your Time Machine backup is backing up both the boot volume and the SD card before you delete things, but I've had no issues running OS X like this from 10.7 Lion through the betas of 10.11 El Capitan.
Some apps are cranky, so I would leave all system files where they are and worry about things that are 10 GB or more in size rather than trying to micro manage dozens of small relations that don't yield significant storage savings on the boot drive.
In your case, you can use iTunes to locate the iTunes storage on the external SD card and moving the iPhotos Library is trivial as well. Move it from Finder and then open the new file. Delete the original file once you're sure things work and/or you have a solid back up.