I've had my lads done early in the past by a month or two when they were younger and there were no problems. If you're really worried, have you tried contacting the vaccine manufacturers (your vet will probably be tied into one or the other of them) to ask if they consider it safe? I emailed Merial about their rabies vaccine and the woman who answered was great, v helpful. Are you talking about the cat flu vaccine or the 'nastier' ones (leuk etc?).
Alternatively you could just start the AXA insurance now and wait a month or two longer to vaccinate the boys? As indoor cats there shouldn't be any worries doing it like that surely? Unless you are doing passport vaccinations of course in which case they have to be done before the expiry date or else DEFRA freak out.
How are they - are we getting more photos soon? (of them begging to be fed by their mum
).