Wildflowers like a poor soil, so germination in any container compost may be patchy. Most other flower seeds will germinate so don't sow too thickly.
I'd agree with Dave - wildflowers don't really lend themselves to containers. If you have an area of low fertility where you can sow those that would be better. With the other seeds, it depends what they are - I think you'd have to organise them according to colours and sizes etc to make it work well.
Germination seems to vary between species too Lorraine. I always get good rates with cornflower and corncockle, but corn marigold - not a single seed has come up.
I find that most seeds do best sown in small (40 module set) cells and plant out when they have a well developed root system. Corncockle usually germinates reliably in open soil though.
Sorry to disagree, but I find that growing wildflowers in containers is sometimes better in some ways, especially if you just want small areas. I grew some black knapweed in open soil but the plants simply grew too tall and the flowers too small by proportion.
I half-fill tubs/pots with stones and, if I have any, bits of polystyrene for bulk, then fill the remainder with garden soil only, possibly with some grit added. This has proved ideal for knapweed and ox-eye daisies. This year I've planted birds foot trefoil, yarrow and "rabbits tail" grass seedlings into a trough with a thick base layer of stones, then soil with plenty of grit added. I've topped it with blue slate chippings and an irregular shaped piece of broken roof slate.
Some plants are so evasive when planted straight into the garden, by all means do it when or where you have room to spare in the garden, but otherwise....