Do I need to keep any cautions in mind when transfering an external SSD into my Macbook?
Nothing that I can think of. 

My own experience was just - clone, test for a few days, decide all was good.
Then, eventually retire the old drive for good.

You might want to check that the new drive has the correct Recovery Partition in place before making the final swap.

Disk Utility can tell you if the partition is present, if you set the Debug menu to 'show every partition', but I'm not sure how to tell whether it's the Yosemite recovery partition.  

Booting into it with no other drive connected would be the certain way; assuming no-one else comes up with a nifty way to ask Terminal etc.