Hard to know whether the situation with Katie and Bubs is all down to the kittens, or down to the ferals who frequent your garden too, and you may have two separate situations going on, with a combined stressor on the indoor/outdoor cats.
With regard to feeding, if you free feed at the moment (and I think you do, as you said they all get fed at different times during the day) you may have to be firm, and withhold free feeding, making them all eat at the same times each day - though not necessarily within sight of each other. You need to associate the older cats with anticipating good things when the new kids are around. The risk exists however that the older cats, being indoor/outdoor, may choose to find somewhere else to eat instead.
As an alternative to feliway, you could try Pet Remedy plug-ins. A few of us on here have had good results with them.
Alternatively, you may have to accept that the older cats may never feel settled with the younger cats, and will choose to spend time outdoors or on their own more.
We found that when we had Paddy and Flynn, Paddy (our older cat) never got over the horror (in his view) of having another cat inflicted on him.
Over the years, he grew to tolerate Flynn (Flynn on the other hand adored Paddy and just wanted to be loved by him in his turn) and would occasionally let him sleep alongside, or would roughly groom him, but Paddy was always happier as an only cat, and when Flynn went missing, Paddy began to return to spending much more time indoors.
He was, quite simply, happier alone. That was very sad for us, as Flynn was such a love - a gentle and generous spirited cat. We never did find him after he went missing, but the positive that came from his devastating loss, was the return of our old Padster - playful and loving, content and a lot happier to stay in the house.
As Liz has commented on her very mixed and substantial cat family, they will learn to get along, though they may never love each other. A group of cats will always establish an accommodation - not a hierarchy in the way that dogs will, as cat communities have a habit of changing dynamic from time to time.
With our lot for example, since Visitor Cat No 1 had his road accident, he's treated differently by the others, and often gets picked on and shoved around a bit by all of them. But - just as often, he can be found cuddled up alongside the others - even formidable Boss Cat Tinks.
In many respects it's still early days with your crew, and when the colder weather comes, the older cats may choose to spend a bit more time indoors. I know how upset this will be making you feel but I still dont think you're doing so badly to be honest
