which Dr brown nipples?

Ok, at 6 weeks old what size Dr browns nipples were your babies using? My son was a 35 week preemie, used the medela bottles and nipples in the nicu and started with the preemie size but was up to slow flow by the time we left at 2 weeks old.. but after a week or so at home it was too slow, it took him 45 minutes to an hour (sometimes longer) to eat 2oz.... we then switched to Dr Brown's bottles with the size 1 nipples and he did great, 15-20 minutes and feeding was done..... He is now 6 and half weeks old and I'm starting to see him slow down again and having some gas issues...we are at 30 minutes for a bottle now (3-4 oz)... I tried the size 2 nipples and he took his bottle in 15- 20 minutes... I do have to pace him a little in the beginning as he gets excited and sucks too hard when we first start. but once you get past that he gets the right rhythm going and does fine after that.... the size 2 nipples say 3+ months... He is only 6 weeks.... is this ok or should we stick with the size 1 and just deal with the longer feeding times? I don't mind... I just don't want him burning more calories trying to eat than what he is taking in right now... weight gain is our main priority as he is on the smaller side and had some catching up to do and he doesn't eat a whole lot at once or even eat frequently... he is totally content with 3 oz every 4 hours...