Fact: Not all stitch patterns knit at the same gauge. duhhh..
Yes, we can mix 'n match stitch patterns, but there are considerations.
It's tempting to plan to put a different stitch pattern up the sleeve of our cuff-to-cuff baby sweater or a section of a sweater. But .. consider this ...
If there is a considerable difference in gauge (especially row gauge) the center section will be "wonky".
OK - that's not a technical term
In this example, our stockinette gauge is 6 rows to 4" and we knit 24 rows in stockinette to knit a 4" piece.
Let's say I chose a stitch pattern for the center that is 9 rows to the inch.
I knit 24 rows in pattern.
Holding up the 2 pieces you can see that the decorative section is much shorter and would have to stretch quite a bit to match with the stockinette piece.
Another way to look at this ... the longest length of my sweater is 20".
Knit at my stockinette gauge I'll need to knit 120 rows.
Knit 20" at my pattern gauge, I'd need to knit 180 rows.
That's a big difference if you are knitting them both at the same time
What's the solution?
If your gauge isn't as extreme as this example you could mix stitch patterns and gauges, and block the piece into shape ... it's knitting for heaven's sakes, not wood working or diamond cutting that require absolute precision.
Hand knitting patterns do this all the time. They rely on blocking to even out the differences.
Because machine knitters have easy access to more stitch patterns (with a variety of gauges) it's important to understand the impact of mixing and matching stitch patterns and gauges.
Swatching Is NOT optional!
Another solution is to knit panels. In my example, I'd knit the fronts and backs at 120 rows, then knit the center section at 180 rows.
Neatly seam the panels and even the most extreme gauges will match up.