How can I get 5-month old twins to take longer naps and feed less often?

My twin girls are on a pretty rigid schedule (set by themselves, not me) and according to all the books I've read and details given on infant formula sites (Enfamil, etc.), my girls are feeding more frequent than expected and are taking very short naps. They feed every 2 1/2 to 3 hours, and maybe twice a week they'll do a 4-hour interval in the late afternoon. They get 6 feedings a day (combinations of 6 oz. of breastmilk/formula or 3 oz. breastmilk/formula and rice cereal + baby food). They sleep 7 to 9 hours a night, depending on the time of their last feeding, and will always wake up at 6:00am on the dot. Their naps span anywhere from 30 minutes to 1 1/2 hours, but mostly at 1 hour, no matter how long they have been awake since their previous nap (typically awake for 2 hours). They are very happy babies and I know they're getting enough food, and based on their personalities it would seem that they're getting enough sleep, but I find that hard to believe considering they're still so young. I know that every baby is different and that I should take information from these books with a grain of salt, but it's difficult to ignore them when several sources agree on the same feed/sleep schedules. For example, Enfamil states that 5-month olds should be eating 5-7 oz. every 4-6 waking hours, equating to about 4-5 feedings a day. So am I doing something wrong? I try to push the girls to do longer intervals between feedings, but when they wake up so soon before their next feeding it's hard to keep them calm for longer than an hour. And I don't want to let them cry it out in case they're legitimately hungry. If I could at least get them to take longer naps it would allow me to get a lot more done in the day. Thanks for your suggestions.