If D3D11 is just a superset of D3D10, and there's no fixed function pipeline for D3D11... My question is, would I be able to use D3DX10.h in conjunction with D3D11 to use interfaces like ID3DX10Mesh and ID3DX10Font in my game?

You can use ID3DX10Mesh indirectly, but you cannot use D3DX10FONT.

The mesh works because you can extract the raw data for D3D11 to use, without any requirement of using the D3D10 API. The data layout is the same for either, and where necessary you can easily write a translation.

The font stuff doesn't leave you any option since it's coded directly to the D3DX10 API. D3DX10Font was dropped in preference for using Direct2D.