It is practically possible to manage some cattle with gates but (a) I have had Dexters and although they are small, I would not want to put an AI operative or a vet in the position of handling one with only gates to constrain them, and (b) many vets will not now attend cattle unless you have a basic crush, including, if you are in England or Wales, the vets doing your regular mandatory TB test.
Vaccinate or not is about risk factors for those cattle and your location, so talk to your vet.
Ivermectin is a broad spectrum wormer and ectoparasiticide. Unless you have a problem with lice, mites or worms on your land, or these animals have arrived with them on board, adult cattle would not normally need to be wormed. Calves still suckling their mothers are also generally regarded as at low risk but would normally be wormed once on weaning, or on the next turnout after weaning if being weaned at housing.
Fluke may be a problem in your area and if so, all cattle need to be treated every year. Again, your vet will advise. If you need to fluke and also want to give ivermectin, then Ivomec Super has a flukicide alongside the ivermectin and is what a lot of cattle farmers use.
Fluking is nornally done on housing with an in-wintered herd, or would need to be done at least twice in autumn and winter with an outwintered herd.
Note that ivermectin cannot be given to any cow whose milk is intended for human consumption whilst she is lactating or within the 60 days prior to calving. With dairy cattle it is usually therefore given once a year on drying off at 60 days before calving. (I mention this only because some smallholders start with Dexters with the idea that they might milk them for the house. If you were thinking along these lines, you might wish we'd mentioned the thing about ivermectin and milk for human consumption

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