"Can you have Macs and PCs on the same wifi network? Can they use the same wireless printer?"
Yes, you can have both on one wireless network. The "language" that computers speak to each other on networks (TCP/IP) is supported by both types you mentioned. Heck, my house has one network that handles three PCs, two iPods, a Mac laptop, a Wii and a Linux machine. For printers, as long as the PC has a proper PC driver for the printer and the Mac has a proper OSX driver for the printer, they can both use it. You can get both drivers from the website of the company that made the printer.