I formula fed my son and I only switched to one special formula to try to rule out milk allergies.. Other then that, he's on one brand and then I'll switch him to Organic Milk around 11 months old..
I know they make special high calorie human milk fortifer for low weight babies.. I've seen those at work, but only used on babies that need the extra calories..
Here's an example (these are vary expensive too.. It's crazy!)
Similac® Human Milk Fortifier