You can, but fusing drives can cause issues, especially if you ever want to remove the external drive. If you ever plan on removing the external drive, or even accidentally remove it, it could really mess up your computer. It may be better in the long run to keep them separate, and then you can pick and choose what goes on the SSD. An 840 EVO or a Crucial M500 should be more than enough for your needs. The only time you will notice a difference between those and the 840 Pro is if you are constantly writing data to your disk. The 840 Evo uses a system where it has 500MB of really fast storage to write onto, and it pushes it off onto the relatively slower (still faster than many SSDs) storage once it is done writing. The 840 Pro is all really fast storage, but it will be no different than an 840 EVO for 99% of people. Do you write 5GB or larger files via TB or another high thoroughput connection on a daily basis? If not, the 840 Pro will be as useful to you as an M500 or an 840 EVO.
I would recommend getting a TB external drive so you can enable TRIM, and a 256/512 GB 840 EVO or M500. Load the OS onto it, and manually push files to the SSD as needed.
Here is a link to help setup OSX on an external drive. You should be able to setup Bootcamp just fine on it as well once you install OSX onto it.
http://support.apple.com/kb/HT5911?viewlocale=en_US&locale=en_US
Matt