Booting ubuntu on macbookpro with osx and windows installed
Yes. You are right.

rEFTit will let you choose what to boot, including the ubuntu. 

Also, not many people know this, but fsck tends to break journaled file systems after alot of usage (not sure how this applies to SSD's).

So, you did a good thing not installing the *nix based operating system on the same drive as the mac.

Here is one worry I have. Im not sure if a reEFIit knows how to read (boot) a thunderbolt drive.

Here is my setup:

2013 Air 512 GB SSD split into 3 partitions
rEFIT has replace Apples original EFI. 

sda1 -&gt; Mac OSX (Never boot into unless for firmware update)
sda2 -&gt; Windows (Almost never boot into it unless doing something for work)
sda3 -&gt; sda3a sda3b sda3c
sda3a -&gt; Linux Mint
sda3b -&gt; Arch Linux
sda3c -&gt; gentoo. 


rEFIt gives me the first 3 options. Then I use grub to choose my flavor. 

Hoe this helped.