Kittens' gums are inflamed

The kittens we rescued are now 5 mos old. They still are getting adult teeth in and still have some kitten teeth. But I am concerned their gums are already varying degrees of red. I am working on getting them used to tooth brushing. I started with one of the boys and he did great till the brush touched a tender gum by a molar and bled. I hardly pushed. Then he had had enough. I feel sad because he was so okay about it up to that point.

Is this Inherited periodontal disease at five mos?! Teething?

Also, we had them tested for parasites the first day we found them and they came up neg. but we have discovered recently a roundworm so they are on meds for that. I don't know if this would have anything to do with their teeth though. They look very healthy otherwise.

Can anyone help with this to explain what is going on with their teeth and how I can still help them by brushing? I have found with our last cats, it is so important to get them used to brushing early or they resist it their whole lives.

They are at the age where teething is very common and they start getting their adult teeth, so I'm going to go with teething.  However, if you're really concerned about it, ask the vet at their next check up or appointment for kitten shots.