100% they can all be on two feeds a day now. They will eat more creep if they are getting less milk, which is a good thing.

. Even the youngest can make some real use of the creep now, and the older ones have fully functioning rumens so will make good use of the creep.
The thing about older lambs aspirating milk is if they didn't get presented with the bottle correctly in the first few days, the oesophageal reflex (which makes a closed tube to deliver milk only into the abomasum) may not have developed correctly. Milk therefore sloshes about in the throat and can end up in the wrong places - windpipe hence spluttering and foaming, and potentially also rumen which can cause bloat.
You can't make the oesophageal reflex happen now, so your best choice is to feed less milk rather than more, and wean that lamb as soon as you are sure it is eating creep well. Ideally at least half a pound a day, but it's hard to know how much individuals are taking when they're together with ad lib cake, so you can only guesstimate.
If they were mine, anything 8 weeks and older that's eating creep, I would wean now. Anything 7 weeks and older that foams or coughs when bottle fed, also wean now if it's definitely eating creep, otherwise go down to 1 bottle a day (and not too big a one), and keep an eye on it to make sure it eats creep to make up for the milk it's not getting. Then stop the milk when it's definitely eating creep.
The 6 weeker, I would go to 2 bottles a day now, for another week, then stop as above, ie., sooner rather than later if it's coughing when fed, but make sure it's eating creep if you can.
I wouldn't be blasé about the possibility of bloat, but also I wouldn't let it terrify you. Bloat doesn't kill them in 20 minutes, and it usually responds well to prompt treatment - and should not require the vet if you spot it and treat it promptly.