Most dewormers on the market are approved for pregnant and lactating mares, foals after a certain age (age depends on the drug used) and breeding stallions. You would need to check the label on the dewormers to get that information.
However, there are times when deworming is not necessary due to environmental factors which make reinfection risk extremely low. For example, when temperatures are above 85 degrees or below 42 degrees. Both of these temperature ranges make it unlikely that there are going to be high numbers of infective strongyle larva in the environment thus the risk of horses getting heavy parasite burdens at these times is low.
I would recommend that you watche the deworming webinar available through The Horse | Videos