Yes you can use swine formula just make sure you adjust the dosage so you dont over dose.Why are you rotating between the two?
If you are using ivomec for heartworms it should be done once a month. And the strongid if used for intestinal worms should only be used if they have worms.
Heres a link that was already started about ivomec and its dosages. Hope it helps yopu a little.
http://www.coondawgs.com/forum/viewtopic.php?t=18085