I only have one desktop in spaces, and when I take an app full screen, it always goes to the right of my desktop... I know I can re-arrange by hand in mission control, but I'd like to be able to skip that and have an arbitrary app open to the left of my desktop.
If I have understood your question correctly, you can achieve this by the following:
1. Launch Mission Control 2. Add a new desktop by clicking Add to the right 3. Drag the desktops in order of preference 4. Exit Mission Control 5. Navigate to the desktop you want an app to open on then right-click the app icon in the dock, choose Options and select This Desktop underneath Assign To (this assigns the selected app to the active desktop in either full screen or normal maximised views)
Note that when you drag the desktops, the naming will always be ordered numerically from left to right. So if you began with a single desktop, adding any subsequent desktops will change the name of the original desktop to Desktop 1. If you drag another desktop to the left of Desktop 1 then Desktop 1 will become 2.