Pumpkin, squash and zucchini all belong to the same genus, namely Cucurbita. There are four main species:
Cucurbita maxima - pumpkins like Queensland Blue
Cucurbita moschata - pumpkins like Waltham Butternut
Cucurbita mixta - pumpkin like Green Striped Cushaw but this species is not common in Australia
and finally
Cucurbita pepo - pumpkins like Winter Luxury Pie, squash like Lemon Squash, marrows and of course zucchinis.

There is some doubt as to which will cross with which. They will certainly cross within a species so a variety of zucchini will cross with the pumpkin Winter Luxury Pie or any marrow or squash. Cucurbita mixta will cross with Cucurbita moschata but neither will cross with the other two according to most authorities and Cucurbita maxima and Cucurbita pepo will not cross with each other.
What this boils down to is this. You can happily grow Queensland Blue, Waltham Butternut and Lebanese zucchini together and be pretty certain that if you bother to collect seed for next year, you'll get the same again.
If you grow two varieties of zucchini and a squash side by side, then seed saved may well be crossed so what you get next year and subsequent years will be a surprise.
If you're not collecting seed then it doesn't matter what you grow where.
Hope this helps and sorry for the first aberrant post.
Ray