
If you're going to let them out you will have a much less chance of taming them down....... The smaller the area they have the better really. I.e. for months just one room, then increasing it as they gain confidence. Personally I would never let them out of course, but even more so in this case. I bought in two adult cats last November, one was nervous -but certainly not semi feral, she's a persian. She was in hiding indoors for months and months.We never saw her. It wasn't until now when she's had kittens and has spent 10 weeks so far in one room only that she has finally settled enough to allow herself to be picked up easily without panicking. When she was still able to run off and hide, she did it every time, not having the possibility to run off is what settled her, and I have seen this a few times.
The stuff you were told about not rehoming two kittens from two different litters is total rubbish. :( There's no reason at all for that, kittens will mix perfectly happily and there's no better time to introduce them.
I'd say you need to think of what matters most to you. Rescuing a couple of kittens and giving them a good life, or having cats that will be people loving. Moggies are always pot luck compared to pedigree cats, just like with dogs, but even more so of course if you're talking feral kittens. I'm not at all anti moggy, indeed I show and judge mogs and ALWAYS have at least two rescues at home, and I even wrote a book about how great moggies are. :) But for the first ten years or so as a cat owner I was convinced cats were not for me as my experience was that they're not very friendly and loving, and it wasn't until I saw the difference in pedigree cats that I realised just how different they can be. Rescues can become fantastic, but it will take more time, and you have to be aware of the fact they may never be lap cats.
Like Christine said, you need to blood test the kittens for Leukaemia and FIV, especially if they are going out -but be aware ANY cat they meet outdoors and have a fight with may be positive for FIV and that is the one fatal disease you cannot vaccinate against and which can be carried unseen for years before any symptoms show.It is spread via bites.