mickey, you're going to find that your videos kind of suck without a gimbal, and it'll be a challenge to set up FPV. the main point of zenmuse gimbal is to smooth out all the vibrations and other movements of the Phantom and create stable video (as well as carry video signal back to the Phantom mainboard). without a gimbal every raw shake, shudder, and lateral movement of the phantom will translate to wild shaky sickening video from the gopro. for example when you move forward and the Phantom tilts nose-down your camera will now be facing down too. that's not so bad, it's all the other vibrations and such that will be rough. you may want to look into cheaper gimbals, I feel like I've seen P2 gopro gimbals for like 150 usd on ebay as opposed to a Zenmuse for 300.
anyway to answer your questions yes, it's super easy to just add a gimbal later... however unless you have a zenmuse which both powers the gopro and takes video signal back to the Phantom mainboard, you won't be able to simply plug-and play with the pre-made cable, those cables are designed specifically for if you have a Zenmuse and get your video signal from the Phantom mainboard. If you do NOT have a zenmuse, you're going to have to snip and solder some cables to patch your video input directly from the gopro.
Assuming you get
this cable, you'll also need to get a
GoPro USB video cable like this one. You'll snip the connector off the gopro cable leaving you yellow/black wires. You will then have to manually patch that yellow/black pair into the yellow/black pair with the WHITE CONNECTOR from the firstpersonview cable but you cannot simply cut off the white connector! You still have to plug that white connector into the Phantom main board to get power for the video transmitter. you only want to interrupt the video signal (yellow/black) and patch in your direct cable. make sense?
If you're on a budget, consider the Boscam RX-LCD5802 receiver. It's about $50 cheaper than the Black Pearl and is a full 32ch 5.8ghz diversity receiver. I don't have the IRC transmitter but I've heard from some folks here using the IRC and RX-LCD5802 and they say it works fine, better than the 32ch Black Pearl which has issues.
last question, if you then buy the Zenmuse gimbal later you'll either have to re-patch the firstpersonview cable back to the way it originally was... or just buy a new one