Brahmas take longer to mature. You will not get eggs as early with Brahmas as you would Wyandottes.
Since Brahmas are larger, the feed to egg conversion is not as efficient as Wyandottes. Same for feed to meat conversion if you are interested in eating them. If they are foraging for food instead of you buying the feed, this probably does not matter to you.
Could you get hatching eggs and buy or borrow an incubator? That would solve the mixing flocks problem.
You can get chickens from different sources and combine them. People successfully do it all the time. Some quarantine and some don't. In some circumstances I strongly recommend quarantine. In some circumstances, I think it is a waste of time and effort. Let me try to explain.
When you quarantine a chicken, you are looking to see if it develops any symptoms of disease or infection by parasites before you put it with the rest of your flock. The parasites, like mites, lice, and worms, are always a threat to your flock. If you keep chickens long,you are highly likely to see them. They can get them from wild birds or eating earthworms, snails, or slugs. If your chickens get mites, lice, or worms, it is not life-threatening as long as you pay attention and treat when they show up. I rate parasites as certainly possible they could br carrying them but the consequences are pretty low as long as you treat when the symptoms show up. Whether you quarantine or not, you should always observe closely when you combine the birds.
A chicken can have a disease, be a carrier for that disease, yet have an immunity to that disease. Coccidiosis is a great example, although there are others. If a chicken has the cocci protazoa internally and has developedan immunity to it, you can keep it quarantined forever and never know it. The only time you will know is when you combine the flocks and the other chickens come down with it. Or if a chicken has been vaccinated for a disease, say Marek's, it can be a carrier but never exhibit the symptoms of that disease. In these cases, quarantine is of no benefit. If you do get chickens you should ask for a vaccination history, by the way.
What you are most likely to gain out of quarantine is, if the chicken has recently been exposed to a new disease or infection in a recent time period, you may catch that disease before you introduce it to the rest of your flock. If the flock the chickens are coming from has not been exposed to a potential infecting source, the odds of you finding something in quarantine is pretty low. There is a risk that you might find something by quarantine, even in a chicken from a closed flock. I am not saying there is no risk. The chickens may have been exposed to something during shipping or a wild bird may have brought in an infection.
If a breeder has introduced new chickens to his/her flock in the last couple of months, has taken any of his/herchickens to a chicken show, or exposed them to other chickens at all in the last two months, I think quarantine is certainly called for. If the chickens are coming from closed flocks where they have not been exposed to outside chickens fro a couple of months, I probably would not quarantine. There is a risk but I think the risk is low and the potential benefits of quarantiine, for me, are not worth it .
When I add new blood to my flock, I'll use hatching eggs, not living birds, mainly because of the risk of introducing diseases.
