You can put the plants together if you want. But the pot needs to be 8-10 gallons and they will be fine. I have all my plants together in large pots packed with Nutrients. When they all grow together you will have a nice Bush that will fill out.

So to start your using bag seed, well who ever told you that about the size of the seeds is wrong, Say if you had some named strains and if they where mixed you be able to tell what seed is from which strain. Say Like Trainwreck that is a clone only plant, when you cross it the seeds are really small. But you have to known the strains in order to figure it out if they where mixed.