First of all I would ask if Mustache's FeLV status has been confirmed with an IFA test which would indicate whether or not he is l ikely to remain persistently viremic.
Secondly, I would adavise the person who is caring for Mustache NOT to give him any vaccinations...this will only be an assault on his immune system that he may not be able to withstand. Repeated vaccination with combo vaccines is not good for healthy cats and there is no point in giving vaccines to a cat who is immunocompromised. The best thing to do for cats who have a compromised immune system it to keep them indoors and away from cats who may go outside and bring pathogens in to threaten their immune system, keep all types of stress to a minmimum and feed them a healthy diet with immune support. Unless the shelter allows the cats to run freely in groups, the other cats at the shelter are at no risk due to Mustache, while he would be at risk from exposure to them. The FeLV virus is not airborne and transmission other than from repeated, direct contact wirh a cat in stage 4-6 who is actively shedding the virus is highly unlikely as long as reasonably good hygiene (washing hands, food bowls, etc.) is practiced. It seems that whoever is caring for Mustache needs to learn more about FeLV and caring for a cat who is immunocompromised so that he/she can better advice potential adopters. Is there anyone on this list in their area who might be able to help in that regard. If people were better educated about FeLV, it might be easier to find foster homes and/or people willing to adopt them. Sally in San Jose
