For the purposes of laying-out a garden to minimize cross-pollination, would I be well-advised to, for example, plant watermelon (genus: citrulius) between my butternut squash and acorn squash (both genus: cucurbita)?
Yes, that would be a good plan.
If you’re not going to save the seed, you don’t have to worry about cross-pollination. You’ll get what you originally planted.
If you want to be ‘professional’ about it, because you want to save the seed, you can use what are called ‘Isolation Cages,’ info, below:
https://blog.territorialseed.com/blog/2017/2/15/isolation-cages-for-the-purity-of-seed
Also pinging some of our ‘experts’ on saving seed!
Butternut is C. moschata, while acorn squash are C. pepo. They won’t cross no matter how close they are.
If they were both C. pepo or both C. moschata, then they would cross. And in that situation, putting a watermelon between them would not be enough! On rare occasions C. moschata will cross with a C. maxima, but not often.
This is why I’m able to grow 5 kinds of squash in the same field and still keep the seeds pure. As long as each is from a different species, and the maximas and moschatas are at opposite ends, they won’t cross.