I would not plant the seeds in the pots. I would start them in a flat of some type. The reason for that is that you are using mixed seeds. That means you'll be growing several different types, some of which will sprout very quickly, but others of which will take much longer. Using flats, you'll be able to very gently lift out the newly sprouted cacti seedlings (I like to use the pointed end of a plastic plant marker for this purpose) and plant them in the pots. I think I would just transplant them to a separate flat, however, and wait until they're a little larger to put them in the pots. That's just my own opinion, though, so others may prefer other techniques.
You may need to use a cover/humidity dome of some type, but fungal infections, such as damping off, can present a major problem when starting cacti from seed. (I find myself fighting too much humidity in my home during the winter and spring months, so I never cover seeds of any kind when germinating them. I find I have better control if I just use a fine mist sprayer to keep the soil surface just barely moist. Your situation may be different, however, so use the covers if you need to.
) You will want to get your little baby cacti out of the humid environment as soon as you can, which will be impossible if you do the germination in the pots.
You may also need to use something as a source for bottom heat. I use old heating pads, set on the lowest temperature, for this, but I also have an old waterbed heater which works extremely well when set on its lowest temperature.
Here is a link to an Extension Service article
in which you may find additional useful information:
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