When rescuing ferals, or strays, it is my opinion that you go straight to the vet FIRST. They need to be health checked, wormed, etc before coming into your home. Especially if you have other cats. Some illness are airborne so it matters not if they are separated. Some germs can be carried on your clothing from you being where the un-vetted cat is staying.
The vet knows how to take care of cats that are feral.
Besides, if you wait to take a feral/semi feral to the vet AFTER you gain some trust it is very likely that will cause a set back and you will be starting over.
Getting spayed/neutered now will decrease problems that will occur if a female goes into heat (starts yowling and trying to get out to mate) or if an un-neutered male starts to spray to mark territory and seek a female.
I hope that made sense. Bless you for taking care of them. There are several good threads regarding socializing ferals right now. Give this blog a look www.feralcatbehavior.com . Much advice there, just do a search.
I have been reading everything I can get my hands on from this site and externally for the socialization process of these cats. I know it will be a long process and we will have setbacks, but at least we know they will make it through the winter.
Lori, Rick, Shadow, Squirt and now Baby