Yea, they could need worming again in 6 weeks. Although the pasture hasn't been used since last autumn, and some of the dormant worm eggs will have hatched and died since then, not all of them will. And anyway, when you worm an animal you don't kill every worm, so they will have still been adding worms to the pasture.
Garlic - hmmm, somewhere between an old wives' tale and a country remedy I suspect, ie might help a bit, unlikely to do much harm, but probably not overly effective.
Apparently we should use one class of wormer til we see evidence of resistance, then change, swapping all the time will just mean you get resistance to all of them. Also the Moredun folk say worm and don't move immediately to clean pasture otherwise the only worms on the new pasture are by definition ones that were resistant to the wormer and then they have no competition - the field to themselves, so to speak