I believe that it has been 'suspected' for some time that it is NOT a good idea to use ONLY invermectin for deworming, at any age. I certainly would NEVER use ONLY ivermectin to deworm, especially young stock!
Pinto28, it should be OK, time-wise, to deworm your horse again, but I would not recommend using Equimax, which is a combo of ivermectin and praziquantel. If roundworms(ascarids) are becoming resistant to ivermectin, giving yet another ivermectin dewormer this soon, even in combo w/ praziquantel, which is basically only to get rid of tapeworms, is probably not the best approach. I would recommend a fenbendazole--Safeguard or Panacur--and as recommended in the article in the link provided by Kellie in OR.
Various kinds of rotational deworming have LONG been recommended--though sometimes, it's been thought best to use 'every time' dewormer class rotation, and at other times, to use a 'slower' rotation, using the same 'class' for as long as a year, then switching to another 'class' for the next year, etc.....I follow my own instincts/experience, and have sometimes used the same 'class' twice in a row, but never more-then change 'class'--again, for never more than two in a row, at least for adult stock. I also prefer to use a 5 day 'purge' for encysted small strongyles once a year(sufficient under my particular personal circumstances), and a 2 day 'purge' for tapes once or twice a year, using the recommended dewormer classes for those worms. I do not use any form of moxidectin(Quest) for minis, nor do I use any combo w/ praziquantel....out of personal choice, because I believe the purges work best.
Your best bet, for you personally, would be to double check w/ your own vet, who would know YOUR circumstances, and could verify what should be the best program for YOUR horses.
Best wishes,
Margo