New and old guinea pig questions

Question
I foster guinea pigs for the RSPCA, and at the moment I am fostering a boy, approximately a year old, who has long hair (after investigating on the internet he looks like a coronet). I have little experience with long-haired piggies, so at the moment the only care I am giving his coat is a brush once a day. Also I have had this piggy for about 3 weeks now, but still feels very thin. He doesn't eat a lot, but will nibble at most things I leave for him in his hutch. Also, because he is living on his own I wonder if he is lonely. He often gets agitated if he sees or smells one of my female guinea pigs and can't get to them, and I wondered if this might make him stressed. (He is being neutered in a few weeks time and can then be re-homed with another female). Is my handling him good enough company for a little while, or should I move his hutch closer to my girls so they can "chat" to each other?

Also, I have 3 girl guinea pigs of my own living together (they have plenty of room in a large rabbit hutch). The eldest, who is nearing 6 years old, often gets a very dirty behind, which I find hard to keep clean. She has fairly long hair on her bottom which I try to brush, but it gets matted very quickly. I have tried wiping her with a sponge but she does not like having her bottom touched. Often it gets to the stage where I am cutting out clumps of knotted fur that are impossible to brush through! Obviously I don't want to pull her hair as she has sensitive skin in that region. I remember my other guinea pig had a similar problem, but her fur wasn't so long as this one.

Many thanks for any advice.

Answer
I have rescued a lot of skinny pigs too, they do get fat in time it just takes a while because their digestive systems are very picky and eating too much can result in diarrea, which will make them loose even more weight. The best thing to do is just keep giving him plenty of pellets, and also unlimited hay, right now you could even give him Alfalfa Hay to speed up the fattening process, but then switch back to timothy hay when he gains his weight. A lot of males do tend to be a little skinnier than females, especially breeders. He would probably like to be closer to the females, just place their cages side by side, I have had males before that throw little fits if they can't see another pig near by, but they don't like to be in with other males of course just females.
 I also sometimes have the same problem with hair issues with my sheltie female, her hair is around 6 inches long. The best thing to do to get rid of the hassle would be to just cut the bottom hair, this prevents any knots from starting in the first place. If she is having problems with soft stool, which many older pigs do, you can use 1/4 tsp of cherry childrens Kaopectate about once a week to make the stool a little harder, usually guinea's like the taste of it too. There is not much you can do other than cut her hair and try the kaopectate, she could also have a bath but if her hair gets matted all of the time it wouldn't help, also you can use Johnson and Johnson No More Tangles on her hair. Let me know if you have any more questions.
Brittany