Nice looking boas but dang they are are huge for that age or its the pictures. Rango my little salmon 100% het snowglow boy was born Mar 31st 2016 and he's on hopper mice and about 24". He has been slow grown though. And yeah you definitely want to quarantine. I mean at the least if you have to keep them in the same room, don't let anything between the new snake come into contact with the old snakes and always wash your hands inbetween handling and stuff. Here's a picture of Rango to give you an idea of how much smaller he is.
Oh and honestly, I would feed your snakes mice until they are big enough for weaned rats. Baby rats have underdeveloped muscle, bones and organs and are filled with mothers milk which isn't great for a boa. All my boas get mice until large adult mice no longer leave any sort of lump at all, then they go up to small weaned rats.
Also with boas, they develop the fat around their organs first so by the time you actually see the fat on the outside, it has already been going on in the inside for awhile.