The answer is yes, it is ok to keep cats indoors but there are soem basic requirements. First, it is best to get 2 kittens (for the reasons already stated) and second, you have to make their environment stimulating. That means lots of toys, hidey places, scratching posts, cat trees and other places where they can get up high. Cats like to use all dimensions, so if you allow them to climb up high, as well as hide under things down low, they are happier.
I've got 2 kittens (7 months) and I keep them in for a number of reasons. You have to put in the hard work to keep them amused though. I change the furniture around quite often - they seem to get a new lease of life each time. I read somewhere that cats (in the wild) pounce over 20 times a day whilst hunting. If you can simulate that with cat toys etc, you'll be on the right track.