I am making the round neck jumper in the pattern below. I have included a photo of the four row repeat pattern. The bottom of the jumper do not have ribbing - but are knitted, in the four row repeat pattern, with a smaller needle (size 5mm. The rest of the jumper is knitted using 6mm needles). I am using super chunky yarn - and not the chunky as recommended by the pattern. I will be using 5mm needles for the main body of the cardigan (where the pattern cites 6mm) in order to achieve the correct gauge. I also want a ribbed waistband and sleeve cuffs. So, will I need to knit this using smaller needles - or less stitches? The jumper in the pattern is a loose fit at the base - it will need to be more fitted with a ribbing, I think. https://www.woolwarehouse.co.uk/patterns-and-books/king-cole-5288-sweaters-in-magnum-chunky-leaflet
Have you knit a swatch with the heavier weight yarn to make sure you like the feel of the knit fabric?
You can make the lower cuff of the sweater tighter or looser depending on the number of sts. Try knitting a swatch in ribbing and see if you like the feel of it. Then measure your gauge and scale up to the number of sts needed for the lower ribbing on the sweater, whether that be tighter or looser.