hi - hope I can be of some assitance - DS3 is 6 weeks old and has been mixfed for about 3 weeks now. Variety of reasons really - but that's not the point.
To start with he would ONLY take a bottle from DH - it took about 3/4 days before he'd even entertain the bottle from me - and even then I had to give him EBM. With lots of persistance he now takes a bottle of EBM or FF from me - keep trying at it.
It's really important to keep your milk supply up - I've managed to do it by either
a) feeding him a lot during the day every couple of days
b) expresing plenty
We've been lucky that he hasn't had "nipple confusion"....although he still gets a bit of "teat" confusion (tries to suck it like the breast LOL) especially when he's just woken up.
He loves it - as when it's just me looking after him as often he "decides" which he wants - sometimes I offer the breast and he refuses and happily takes a bottle - other times I offer a bottle and he rejects it wanting breast instead - most of the time he'll just take whatever is offered.
I've found it so much easier being able to give him either, as, like you, I've got older children to think about and it means I can make more time for them if they need it.
As for what your HV said about you not making enough for him - utter garbage - your body will make as mucn milk as he needs.
However, if you want to mixfeed for convinience sake (and I find I DO have more time with the older DS's - as he'd (DS3) would spend 30-40minute at the breast.....and I'd be unable to put him down for a long nap afterwards...he wants to be held) whereas he'll take a bottle in half that time, and making up the bottles doesn't take that long!