Hi,
It depends on the plant. Tomato and Eggplant you can save seeds from with only one plant. Zucchini and Patty Pan cross with each other readily so saved seed would be no good.
Tomatoes you get seed from regular ripe fruit so it doesn't matter if it is fruit now or later.
Eggplant, you need to save seed from over ripe fruit. That might slow down production.
Whether the fruit is deformed or not does not affect the seed saved from a variety.
Remy

I have hundreds of chard, I've got to thin a dozen, that started with a single seed about this time last year.

those all sound complicated....my first seeds I saved were from kale, and arugula - super easy.

first - to get plats like the parent, the toms, eggplant etc would have to be open pollinated, not hybrid. If they are - then having only 1 is easy (so log as no neighbors have different varieties). Tom - i've had good luck just collecting the seeds on the cutting board after I make a sandwich or salad...with the eggplant - let the last viable fruit go all the way ripe.

With the squashes - you would need to make sure that a female blossom was only pollinated by the male on that plant. tie a male and female blossom shut before they open. then a day or so later when they're ready, take the male, pollinate the female, ad tie her up again. then let it mature past eating stage to make seeds....

but you might lose out on some produce this way...as the plant will feel it's done it's duty to make seed, and not pump out the fruits anymore.