aggressive/shy

I've had 2 cats since they were babies.. well about 4-5 months ago.. I brought another cat I found outside in *him and the other cats dont fight.. and they share a room..* but he will hide under the bed and hardly come out. Especially if there are a lot of people here. The only time he comes out and rubs on us is when he knows its feeding time.. and he will just walk around and around my legs. Is there any way we can get him to stop being so shy and scared?? When we bring him out to show people he will bury his face in our arm so noone can see him. and he will dig his claws into us to cling to us. My second problem is about 2-3 months ago.. we got another kitten. We keep them seperate *she stays in the bathroom*, but I let them out to play together.. my cats have stopped hissing at her.. and they have started wanting to chase and pounce and play on her... the problem is she gets so defensive.. she scratched one of my cats nose up the other night and made it bleed. Shes only about 8 months old.. and shes very playful too, just not with the other cats. She also tends to get a little aggressive with us sometimes also. Any advice on how to help her stop being aggressive, and the first cat to stop being so shy? I am trying to find them a new home.. but in the meantime I would like to put her in the room with the other cats.. but im afraid she will cause some injuries.

Although I don't foster cats, I know it is very slow and difficult to change behavior of cats one finds outside as the case of your first cat.
3 of my 4 cats I've taken from the streets. Skinny and Wawa are not as wary of people like Joji who always hides from them. I take it that the three had opposite experience with people and thus opposite reactions.
On the other hand, your second cat's aggressiveness to you may be lessened if you show yourself as her alpha. There are some tips in the sticky above.