Common sense would tell you that you should stick with the established routine you have for your toddler and work the new born's routine around that...although Gina doesn't even consider that that there are other children and deadlines in the mix (nursery/school drop off/collection/work/drs appointments etc).
But she does state that you need to have an end goal in sight and then work your plan backwards to achieve this which I found helpful.
So if you want to have an evening routine in place for new born which dovetails seamlessly with your established toddler routine -- work out what this end goal is for you -it might be that NB feeds at 5.30pm and 8.30pm to allow your toddler to be in bed for 7...working your plan backwards would mean that earlier feeds should be 2.30pm, 11.30am, 8.30am and 5.30am etc.
The key to achieving your end goal is having the discipline to start at the right time and everything will then flow from there and you will end up in the right place at the right time -- so in this example you have to ensure that you do the 5.30am feed on time so that the evening feeds will automatically be 5.30pm and 8.30pm to allow toddler to be in bed for 7.
Worst time for any parent is after 5pm we call it "cranky hour" in our house everyone is tired and as a parent you have the busiest section of the day ahead of you which if doesnt go well can be really miserable with squealing toddlers, colicky new borns amongst the ballet runs, homework, cooking, clearing up, bath, story, bed stuff....
Good luck - and enjoy