Is it possible to backup iOS devices to an external drive?
If I understand correctly, you want to backup your iOS devices to a drive other than your Mac's (because they have small SSDs). It is possible to backup multiple Macs to a single Time Capsule. 

Sadly though, it seems that Time Capsule doesn't support backing up iOS devices (yet. I really do hope this becomes an option soon, and I'm not the only one). Here are ways you can make sure those 50GB in your iPhones don't end up in your Mac's SSDs though.

Photos

In terms of pictures, iOS8 should have you backed up with iCloud Photo Library. If enabled, all your pictures and video will automatically be stored in iCloud, which gives you 5GB of free storage (or 20GB for $1/mo, 200GB for $4/mo). 

If that is not your jazz though, you can always sync to iPhoto, but move the iPhoto library to an external hard drive. iPhoto stores all your photos a single archive, the iPhoto Library on your Pictures folder, which you can move as you please. When iPhoto opens, it will automatically look for the iPhoto Library where it last saw it, but if it can't find it it will ask you were it is, and keep working as if nothing had happened.

Backups

iTunes backups might be a little more tricky, but it seems to be possible to move the folder where iTunes stores its backups to an external hard drive, and then trick iTunes into backing up there.

I haven't tried doing this myself, but here's a post that claims you can do it and outlines the steps to follow, both for Windows and OSX. The instructions for OSX aren't too bad, as long as you aren't too scared of using the Terminal application (Applications/Utilities/Terminal). In a nutshell:


Copy ~/Library/Application Support/MobileSync/ to a new location,
say /Volumes/~Your External HDD~/MobileSync. This directory has a
folder called Backups, which is where iTunes stores backups for
mobile devices.


Note 1: ~/ is an alias computers use for your home folder.
Note 2: Your ~/Library folder is hidden by default. You can open it in Finder by opening Terminal typing in open ~/Library.

Remove/rename ~/Library/Application Support/MobileSync/Backup. We won't need it anymore.
Create a symbolic link to the new location for Backups in your external hard drive. This is basically like an alias in the old location of Backups to its new location. To do so open Terminal and type in

ln -s /Volumes/**~Your External HDD~**/MobileSync/Backup ~/Library/Application\ Support/MobileSync/Backup



If you launch iTunes now it should follow the symbolic link and go on with business as usual, not knowing that in fact it's now working with a folder that's on an external hard drive. 

You'll need to have the hard drive connected to backup, 

Hope it helps!