Sarah,
Some rotation plans but these in different groups, but some put them in the same. The simplest established rotation plan is Roots / Brassicas / Others, in which case you should be OK. Others put Alliums in their own group. In terms of soil, both like rich nitrogen soil and good drainage so I think you'll be OK. Although I'm sure I read somewhere that said onions and beans are not good companions and are antagonistic, but that said, a lot of so called established wisdom on companion planting is often proved wrong and I'm sure lots of people out there are growing onions and beans together without seeing any noticeable adverse effects.
So if I were you, don't worry!