I wish I had found out about the non-stitch marker way before starting. I don't think you can switch once you've been doing it the other way, can you??
By `non stitch marker way' you mean purling the stitches between the two twisted ones, right? Yes you can switch. Those are the ones that will be dropped, so it doesn't matter if they were originally knitted or purled.
That would work only if use an even smaller needle than recommended. And is it meant to be a snug fit, or loose?
The other thing you could do is make the inbetween size by taking the difference in the number of sts cast on, divide by 2 and add that to the small size. Like if the CO for the back is 68 and 84, the difference is 16, half is 8 and you'd cast on 76. Do the same for the fronts.