I started to combo feed at 3 months due to his being diagnosed with failure to thrive due to a low milk supply. I had been feeding him nonstop and tried everything to increase my supply, but it wasn't working. I was working full time since 2 months and pumping twice a day at work, going home at lunch to BF him, but I could never get much of anything with the pump, which didn't help matters because I could only pump enough for 1 bottle and he took 2-3 when I was at work.
When I first introduced formula I would BF him and after we would top him up with 1-2 oz of formula, but my husband would give him a bottle of EBM (as much as I could pump the previous day) and then formula for any other bottles he had to give him. At around 6 months, I quit pumping at work because the frustration of not getting much of anything for the time involved was creating more negative energy then it was worth. I'd still BF morning, lunchtime, evening, and for every night waking (every 2 hours at night) and he got formula from my husband for all bottles. By around 9 months, I started offering a bottle of formula before his bedtime, so that I would have enough milk to make it through all the night wakings. It had started to take over an hour to feed him and I could tell he was still hungry by some of the later morning feeds. This definitely helped my supply make it the night and I didn't want to have to deal with a crying, hungry baby in the middle of the night while I attempted to put together a bottle of formula.
As he started to eat more solids, we dropped the lunch and dinner BF and I was only BF during the night. At 11.5 months we started to mix his formula with cows milk and by 1 year had moved him completely from formula to cow's milk for his bottles and at 11 months he finally started to sleep a 5 hour stretch at night, so I was really only BF him 2-3 times a night and at 1 year + 1 week he was sleeping 8 hours and only waking to BF in the morning before getting up for the day. So, I'm still combo feeding, but down to just the one BF in the morning and cow's milk for all his daytime bottles.
We use the NUK bottles, they seemed to help prevent extra gas and he was always a burpy, spitty-uppy baby. We used the generic Up&Up Target brand formula. Cheaper than a name brand and just as good.