I think this whole 'Charles wants to slim down the family' has become a bit of an urban myth. We don't honestly know if that is his intention, or if it will be his intention when he finally accedes the throne.
I do feel, however, that Beatrice and Eugenie should live their own lives, attending the Trooping/Remembrance Sunday etc. if they want while perhaps having a handful of charities that they support. If I were them I wouldn't even try to make a career as a full time royal. I just wouldn't want the continual carping that we already see about the cost to the taxpayer of the bodyguards, the cars, the helicopter rides and so on. No matter what these girls would do as full time royals they'd still be accused of clinging onto the royal lifestyle. The only reason this kind of comment doesn't happen with the Gloucesters, Kents etc. is that the vast majority of Brits don't know they even perform royal duties. The Duke of Gloucester, as dedicated and hard working as he is, could walk down Oxford Street and no one would know who he was.
It won't be easy for them to find employment here, and it's likely they'll suffer much the same press interference in their jobs as Sophie did. So, if I were advising them, I'd say if they can live to their desired standard from the income they derive from their trust funds, they should volunteer their time in a charity or good cause full time. If they can't, I'd encourage them to work abroad in the US or Hong Kong, or on the Continent, where they'll be able to get a job and do it without having the British press continually going on about how they only got the job because of who they are, or is their employer trying to 'take advantage' of having a royal on the payroll? Because, let's say for example, Princess Beatrice is working for a London bank (unlikely I know) and is attending some work event put on for prospective customers. Someone overhears her talking about how she spent last weekend with her cousin William and his wife in Balmoral. That's an entirely innocent conversation. But the British press will trip over themselves to accuse her, and her employer, of trying to profit from her royal connections. To me, that's just not worth it. So, I'd work abroad.