Windows and mac clients can access the same hosted database with little trouble. There are a few platform specific differences, however, that you will need to handle in order for it to work perfectly.
Windows machines require a bit more horizontal and vertical space to display the same text. Fields and blocks of layout text that look fine on a mac may be clipped or wrap in unexpected ways on the windows machine. A few minutes "tweaking" from a Windows client can usually fix any such issues and if you design the layout on a windows machine in the first place, the mac systems usually wont' have any problems with the text displayed.
Use fonts that are installed on both machines.
Store by reference files in container fields inserted by a Mac user won't be visible to windows users and vice versa. Some developers use a web viewer to dispaly image files or PDF's as a way to get around this. Calculation fields can also be defined to function as a "display" container field that returns the correct platform specific filepath for each client.