Hi,
First off, all your plants must be OP and/or heirloom types. Any hybrids(F1) you grew will not come true even if grown alone.
I need to know what kind of squash you grew before I can answer.
If you grew 2 types of cukes, they will cross with each other. Cukes are pollinated by insects that travel plant to plant.
Tomatoes flowers are self-pollinating and not super attractive to insects so the crossing rate is very low so you can save seeds.
Peppers are also self-pollinating, but more attractive to insects so you can have crossing if grown in close proximity.
Remy

I, too, would love a link to lists of these. I know a lot, but some still baffle me. If anyone keeps lists, please post ! Flowers and herbs, too.