I agree, do whatever feels right for YOU. The doctors and Midwives were topping Sydnee up with SMA Gold from birth, initially because she low blood sugar. So I suppose in one way I was lucky as I had the support for combining breast and bottle feeding from professionals on hand right from the word go. Once I got home and settled I began expressing and have combined BFing and EBM more or less from the word go. From about 3 weeks she was on more bottles than breast, as she was so tiny and I have 'crazy boobs' that squirt jets about the room as I feed, so she was gagging and choking etc. It was hard work to begin with as I had to have quite a strict pumping schedule etc, and pump when she fed etc. It was my community midwife who was helping me, and I'm glad we did, as we weren't stressed any more. Our special few weeks were ruined by conflicting advice, constant worrying over what was 'right' etc but once I stuck my fingers up at the world and got on with it we were all so much happier. I could see how much she was having-a big worry as she was only 5lbs5.5oz, she was no longer gagging etc, I was happier feeding and my partner was happier I wasn't throwing 'wobblies' every 5 minutes!
Now that she's bigger we're back BFing more. She'll have two, maybe three bottles of EBM per day, the rest is from Mummy. It obviously worked for us as my 5lbs5.5 baby is now a whopping 11lbs 10.5oz, and is only 8 weeks old!
So I guess what I was trying to say, in my roundabout, gabbling kind of way, is do whatever you feel is right. Expressing earlier on is hard work, and whilst recommended you wait until after 6 weeks due to establishing your milk supply, it IS possible earlier. However, if for whatever reason whatever you're doing is upsetting you in any way, then change it. Once I focused more on enjoying the baby and less on doing the 'done' thing I realised just how amazing motherhood is.