If this sweater is done in one piece starting with the neck and doing fronts, back and sleeves all at the same time down to the armpits, I don't know if it will work to use both stockinette and garter stitch. As Sue said the row gauges are quite different and the number of rows you would need to get the body would be totally different than the number you need to get the arm hole section. Will that work?
I'm not so sure. That in itself would seem to throw the evenness of the raglan shaping off.
I'm just thinking out loud here (if you can call this out loud
), but I wonder if you could do short rows in the sections of garter stitch to catch them up. I've done that on a border of an afghan to make the rows more even when the body of the thing was mostly stockinette.
I have not thought that out far enough to know whether it is a workable solution. This is a lot more complicated application than an afghan border.
Just a thought.
If you are working top down you need increases for your raglans. I am presently making a baby jacket (it's called) that is all garter and has raglan sleeves, but mine is made in pieces and sewn together. Anyway for the increases it seems to indicate just using kfb (or bar) increase, and they are worked on the RS. But I am not using that increase where I need increases (my increases are to shape the sleeves from cuff up) I am using a simple backward loop increase, one after the first stitch, and before the last stitch of each row. They look fine. I'm just throwing this in in case it might help you know what kind of increase to use. Those 2 work, at least on the sleeves I'm doing in garter.
Good luck. Sometimes a supposedly simple change in a pattern can involve more than we bargained for. Another simple change that might satisfy you and have bumps and smooth would be to do the body part in reverse stockinette instead of garter. Not quite the same, but at least they have the same row gauge.
Let us know what you came up with. Merigold