I would say spacing depends on variety and length of growing season. You are in Illinois, so your season is far shorter than further south. Far enough south, peppers can literally grow into trees, but you might only have, say, 4-5 months.

As far as variety, some are genetically predisposed towards compact growth, while some varieties are large open, and sprawling.

Planting pattern also comes into play, especially if you are seeking full ground coverage to avoid weed competition.
For example, you could plant in a single row and get weeds down both sides, or you could plant a double row of plants staggered and eventually cover the space between the two rows fairly well and eliminate weeds there. I personally mulch peppers and have little weed growth.

Lastly, some people cage peppers, some stake them, while others just let them sprawl.

Does it sound like I'm not giving you guidelines? Well maybe, but the answer to your question depends on these factors, and you'll have to take that into consideration.

In my case, I've started extending my spacing from 18" to 2 ft, but using mulch so weeds aren't a factor. I'm growing few plants that are known for compactness, simply because to me those varieties are only beneficial for home gardeners with little space, I want productivity and see that coming from larger varieties.

That said, one of my neighbors planted some 'sweet banana' peppers last year at 3ft spacing. They grew into honking big bushes which may have actually produced as much fruit as two plants spaced half that distance apart.

This year my main crops are 'golden summer'(lime green/yellow) and 'fat and sassy'(green/red) they are expected to get fairly big. I'm trialing 'tawny port'(green/chocolate/red) and 'tequila'(starts out lavender/purple/red)

Hopefully elrem, who is a pepper specialist with decades of experiences will chime in with some words of wisdom.

Finally, the idea that planting hot with sweet will have any effect on taste of fruit this season is not correct. Though some cross-pollination could take place, the difference would only affect plants derived from seeds planted from fruits which crossed, not the flavor of fruit produced this season.

I practice French Intensive/wide bed growing with all my plants except for caged tomatoes. I plant all my peppers & eggplants one foot apart & it works GREAT. Not only do I not need to use any staking or other support (the peppers support each other), but their touching leaves shade the ground conserving moisture & preventing weed growth. Productivity is out of this world.