Anything that gets into one machine on a network can spread to the other machines. It all depends how good your defences are. In principle, the only problem of adding more machines is that it is more work to keep them all up-to-date.
You need to make sure that each machine is properly AV protected and sits behind a firewall. On average, an unprotected machine connected to the internet will be compromised within an hour.
Since you have chosen a wireless option, you also need to protect against unwanted hot spots.