Yes, you only clip one wing.
I would get mums to have a good look round tomorrow. If it is a fox, I'm sorry to say, he'll be back for the others so extra vigilance necessary. Get Dad to pee round the edges of the plot where foxy is likely to enter (seriously).
Introducing a new chicken to an existing flock can be really difficult. Particularly at this time of year when they will be spending more time in the hen house. You are at risk of the new chook being pecked by the existing ones. The "pecking order" can be quite important. Probably best to do this in late spring. You will probably not get point of lay chickens at this time of year anyway because they go off lay soon.
Best thing you can do is buy mum some fertislised eggs and place them under one of her girls (or hatch in a hired incubator). Mother hen will bring the chicks on, you keep what you want (and they are perfectly integrated with the others as having been brought up from chicks) and flog the rest.