Personally I would say separating once fungal shows is too late to prevent it's spread - it can take weeks to show and the chances are that all will be affected once one has symptoms.
Prompt treatment should sort it, Malaseb/Sporal D shampoo for all to control any spread and for a small lesion like this I would use Surolan (if it is diagnosed as fungal). More vets are prescribing Imaverol in cases of ringworm now and I find it very effective.
It could be a small bite wound, scratch from due to mites/lice, so a vet diagnosis is vital.
HTH
Suzy x