Installing motherboard

Should be fine. Just be careful that if there is something to connect that will take some force (CPU fan) that its not one that supports the board there. If so, just install that before the mobo goes in the case. The mobo can take quite a bit of flexing, but avoid it if you can.

If you can get hold of one of the old style nylon standoffs you can clip the bottom off and use it.

I don't like to have the motherboard "floating", it can cause problems when you insert new cards or memory.