We can't speak to toxicity risk since we don't have medical or veterinary training, but with regards to suitability for a wildflower garden, we'd need more information to say whether the listed plants should prosper. Site conditions that are important include summer sun exposure (full sun being 6+ hours a day of direct sun) and soil drainage, plus any needed unpalatability to deer if they browse plantings regularly. The perennials you listed are sun-loving but will tolerate a bit of shade as long as they get at least 5-6 hours of sun in summer, and they have varying tolerances for soil dampness but generally prefer good drainage.

Joe-Pye Weed (Eutrochium) and Rudbeckia triloba can self-seed readily, a good trait if you want them to spread around a bit over time to expand the planting. That species of Rudbeckia is also naturally short-lived, so it will need to self-seed a bit to perpetuate itself as the original plants die off after a couple or so years. Monarda fistulosa can be an aggressive spreader, but if paired with other vigorous perennials, will not take over an area. The Goldenrod (Euthamia) you chose is not the most aggressive of the bunch (a good thing, as some of those are borderline weedy), but it still is a vigorous grower, so having it paired with the Monarda to keep each other's spread in check may be helpful, especially since both prefer moist (but not soggy) soil. The Aster will be clumping and, while it might seed around a bit, isn't expected to be weedy or hard to manage.

We doubt commercial seed suppliers for these kinds of wildflowers would be using pesticides or otherwise not growing the plants for seed harvest organically, even if they aren't certified organic (a step some companies skip since it involves an extra fee to be certified).

In natural habitats, flowering perennials are accompanied by other perennials like grasses (in sun) and ferns (in more shade). While there are some grasses that tolerate partial shade, and some ferns that tolerate partial sun, in general, they are not found en masse together due to their different preferences. Deer, fortunately, tend to leave both grasses and ferns alone. Naturally-occurring meadows that have a stable population of forbs (plants that are not woody, like shrubs) tend to be comprised of more grasses than flowering perennials, somewhere around the 70-30 proportion (70% grasses, 30% forbs). (Although grasses technically do flower, for simplicity, we're distinguishing them from wildflowers with more showy flowers.) You don't have to adhere to this exact ratio, but including some grasses with your wildflowers would be helpful and may support some pollinators by providing them with another food source. (For example, the caterpillars of a few butterflies eat grasses.) For sunny sites in well-drained soil, native grass options include Little Bluestem (Schizachyrium scoparium), Switchgrass (Panicum virgatum, some cultivars of which stay shorter than the wild type), and Indiangrass (Sorghastrum nutans).

There is no set amount of how many species for a given amount of space is best...use what you like that also fits with the site conditions well in terms of the tolerances of those species. You can always add a few more as you see how the initial planting establishes. The species you listed have similar bloom times (about midsummer into early autumn), so adding species that bloom earlier may help a wider array of pollinators and other wildlife. Setting aside any potential toxicity issues (since we don't have that info), candidates may include False Indigo (Baptisia australis), Fleabane (Erigeron species), Wild Strawberry (Fragaria virginiana), Wild Geranium (Geranium maculatum), and Golden Alexanders (Zizia aurea).

Given the quantity of seed usually provided in a packet, if you have good germination rates, you'll likely have plenty of plants to grow in the size of plot you describe, especially if the plants are sown in seed trays first and transplanted as they grow. (You can sow them directly into the ground, but this comes with difficulties of crowding culling some of the plants before they mature, since you can't easily adjust the spacing between seeds, and with distinguishing desirable seedlings from weed seedlings, since seedlings usually look different than the adult plant.

Additionally, bear in mind that native perennials usually have seed that doesn't germinate until dormancy is broken by a chill period (called stratification), and this often requires exposure to cool temps for about 3-4 months (occasionally less). Gardeners can sow such seeds in fall (protected from hungry birds and rodents) and let winter weather give them the needed stratification period, or chill seeds in the refrigerator, then sow them indoors to get a several-week jump start on spring growth. The supplier you buy the seeds from should give an indication of their recommended stratification options and whether the seed is being sold already stratified (unlikely, since they may be more perishable that way).

Miri