Depends on what diseases your older ones have. Do you have anything specifically in mind or just thinking in general?
For certain things it is easier for the chicks to build up immunity while they are chicks, coccidiosis for example. There are probably other things that it might be better to wait to expose them, but I don't know what they might be.
If I am raising chicks in a brooder, I put the brooder in the coop where the other chickens are. I do it for different reasons. I think it helps with integration. It is more convenient for me to not have them and their dust in the house. And it exposes them at a young age to anything the older chickens might have, usually when it is easier for them to build up immunities. I also take dirt from the run and feed it to them to introduce any cocci that might be in the flock to them.
When you integrate them, they are going to be exposed to anything that might be in your flock anyway. I prefer to do that when they are very young.
In my opinion, the best way to keep your chicks safe is to provide them with the correct food, plenty of clean water, keep them at about the right temperatures for their age (I prefer keeping one area of the brooder to that temperature and let the rest of the brooder be cooler. They seem happier and healthier that way.), keep them out of drafts, and keep the brooder fairly clean.
I do not believe in keeping the brooder spic 'n span clean. For them to gain the immunity from cocci that they need, they need to eat each other's poop so if one has cocci they are all exposed. And I don't keep it bone dry. I have one area where the poop can build up just a bit and be slightly damp so they can gain that immunity to cocci. But I do try to keep the rest of the brooder very dry to keep disease down and the area with the poop is kept pretty dry, just slightly damp. A wet brooder is a dangerous brooder and will lead to disease or mold, which will also kill them.
Good luck!