Congrats on your approval! You didn't state which card you got but any amount will grow with NFCU!
There must be something they didn’t like. Do you have high utilization of other cards, high DTI? Navy is very generous if you have low DTI but they become less generous the higher your DTI goes. 18% APR in addition to $700 definitely screams that they find something about your profile very risky.
As for canceling the card, I wouldn’t. As soon as you get approved, you have opened a new account. It will report and your age of accounts will take a hit. I would just use it and then at 91/3 ask for a CLI or second card.
NFCU cards grow quickly, give them a chance to see your "responsible" usage and I'd be surprised if that $700 CL doesn't turn into a $10k one fairly quick.
You might check out the mega-NFCU thread which documents the process from a lot of MyFico members (with NFCU).
I wouldnt look too far into the SL. There is so much more to getting an approval then just a FICO score. I've preached about this in prior threads but I will say that my wife got 1k SL and the reason behind it was her lack of previous CU relationships....smh I know... her score is approaching 700s.
On the forums there is a lot of huge SLs rolling around. I, myself just got a monster but it was after 90 days of treating my first card like a brand new baby kitten. With tender loving PIF care. lol
YMMV but stick with Navy and youll be 30k revolving credit deep with them in no time. Just my 2 cents.