I want to plant several species closely together, low and high closely, then further down the line shift which low/high species are combined/planted. I have about 20 different vegetables to combine in a relatively small plot.
Do you have experience with how close i can do this without it going bad?

What species do you want to plant and what size pot?

No pots, just seeds.
For example radishes amongst garlic, brussels sprouts amoungst strawberries, tomatoes on top of potatoes. like that.

Yes you can, do some degree. Radishes mature very fast, like 21 to 28 days. Garlic is slow. You can absolutely sow radish seeds in between your garlic, more than once. Both those varieties don't have much vegetative growth above ground to shade each other out. Similar examples are beets, leeks, carrots, parsnips. All of those can be companion planted successfully as long as decent space is provided between each variety. Also those aforementioned won't compete very heavily for the available water in the soil. Growing anything too densely results in small produce.

Getting into tomatoes with potatoes, I personally would advise against it. Both those need full sun, and lots of it. The tomatoes (especially indeterminate varieties) will rapidly outgrow the potatoes shading them out, and the roots below ground start going far and wide and will compete for water and one will choke out the other (likely the tomato will win). You will end up with mediocre results. We like to garden to achieve great results with beautiful, delicious, nutrient dense food which puts a smile on our face and provides great satisfaction for our labors involved.

Here's an example of the amount of room I give my tomato plants. I grow in raised beds. Many of them are 4x8 foot, 1 foot deep. I will plant four tomato plants in one 4x8 raised bed. In april, it sure does look like I'm leaving a lot of unused space. I can grow quick maturing veggies (like radishes) in that unused soil in between my tomatoes. But, those tomatoes grow rapidly, and in July, my indeterminate tomato varieties are 6 feet tall, and have started to flop over the top of the tomato cage. By August they've made it back to the ground and are on their way across the ground. They have completely shaded the entire 4x8 raised bed, and drink the water out of that soil like nobody's business. In a perfect world, I would get an inch of rain, steadily over a few hours, every 3 days to keep the soil moist. That never happens so I irrigate every third day.

I am new to growing strawberries, this is my first year. I have them in their own designated raised bed. Everything I've read about growing strawberries is that they need their own space. They are aggressive and will take over. A friend of mine has been growing strawberries for a number of years and said something to the effect of "once you plant them you'll never get rid of them".

I hope this is helpful! Good luck!

The answer to this question is it depends. Plants have certain absolute necessities they need to survive. Different climates provide different amounts of water, heat, and light. Different soils provide different amounts of nutrients imparting different amounts of vigor to the plants.

In my climate it is often beneficial to plant things close enough that they shade each other from some of the summer sun. I am able to kale survive several years by taking advantage of deep shade under trees during the summer. In many climates that same deep shade would be a death sentence. For most vegetables I need to be sure they get the barest minimum six hours of full or nearly full sunlight. I also need to try to extend how much of the day they are in temperatures under ninety degrees, so most 'full sun' plants actually can't take full sun here. Learning how to manage this has been mostly trial and error. It does help to know where a plant originated, both latitude and ecology.

In general, plants with broad, flat leaves can take (and usually need) more shade than those with thinner, waxy, or hairy leaves. Unless it's from a forest or jungle ecology, the closer the plant evolved to the equator, the more likely it is to survive or even thrive under full sun. Plants that produce fruit tend to more sunlight that those we grow for leaves.

It's also possible to take advantage of different growth cycles. I harvest the kale during the winter when it grows well under full winter sun and leave it to nearly hibernate during the summer when the trees shade to create that cooler microclimate essential for their survival.

I grow saffron in another bed. It's dormant during most of the traditional growing season, but comes up from corms in late fall and then grows up through a low cover crop through the winter. As the weather warms I plant warm season crops in the bed and they take over the same space just as the heat forces the saffron into dormancy.

There is a long history of growing 'fertilizer' plants with crop plants. Since many legumes are edible this is actually two supportive crops together. The limiting factor here seems to be water and light. The three sisters garden is a well known classic example which includes a third plant that creates shade in the root zone. Most people say this is to prevent weeds, but I think it also creates a cooler microclimate around all the roots. Plants stop everything when they get too hot.

I don't know if that is any help for you. More specific information on what you intend to plant, and/or where you're growing these plants could help people give you more pertinent information. Maybe there's a permie in your area who's experimented with some of these things.

The longer I garden, the further apart I space things. Rather than asking "how close can I plant...", I'm asking, "How much further can I space them?"  My whole life, I've heard rumors about "companion planting". I don't practice it in my garden. It makes weeding and harvest more difficult for me. I like my rows to be simple, and uncluttered.