I have worked with different weight yarns in the same project before - actually a pattern too. There are several options to create the same “gauge” so to speak. In one pattern it said to double strand lighter weight yarns to make it closer to the heavier weight one(s). You could also go down a needle size or two and knit 2-3 stitches to the one larger stitch (you’d had to do a swatch to see what works). And I think there was another option but I can’t think of it right now. In some projects (not necessarily an afghan though) it looks really cool to use the same needle size with different size yarns. Just my .02 …