I've read that you must simulate winter by leaving the seeds in the fridge with a little damp soil... how long should the seeds stay in the fridge?
It depends on what exactly you are trying to do.
If you want to stratify them, you should have done it in your winter, for approximately 40 days.
If you do it now, you will be sowing in the summer, with is ok but not perfect timing.
On the other hand, if you are trying to give seedlings (not seeds) a period of dormancy, it's much more difficult, and then I agree with Madartej21, try to grow natives first. If you insist on these non-native species, (not all need a dormancy period, check info one by one), prepare yourself for a lot of work and frustration.
Is it possible? Yes. Is it very difficult? Yes.
