I combined breast and bottle feeding from early on and did it for over a year. I feel quite strongly that people don't get enough advice on this. I breastfed for way longer combining the two than if I had been unable to give a bottle and have a break.
My hv was actually really supportive. This is how she advised me.
It's harder work for a baby to learn to get milk from a breast than from a bottle, so if you introduce the 'easy option' too early you run the risk of your baby giving up on the harder option. Some mum and baby combinations figure out the breast really quickly, some take longer. But once you are confident your baby knows what to do then giving a bottle should not make your baby refuse the breast.
Giving a bottle or expressing too early can interfere with supply. So start off by giving the same volume of milk in a bottle each day and let your baby feed as much as they need from the breast the rest of the time. After a few weeks you can be much more flexible and your body will adjust no problem.
The biggest danger is that during a growth spurt you fall into the trap of thinking that your breast milk is not enough and you up the bottle feeding. But in fact your baby spends more time at the breast to boost your supply.
I intended to express and not use formula but quickly gave up on that. Expressing took ages and I hated pouring my hard won breast milk away if for some reason he didn't drink ask much as I expected. So we mixed formula and breast feeding quite happily for months and months.
Sorry, I went on a bit there, but I am a big fan of mixed feeding and I wish there was more information for new mums out there about it.