Do the spectra of compound materials combine linearly with respect to the ratios of the constituent compound spectra?
Yes, you are right. If you have $n$ components in a mixture each with a spectrum $S_1$, $S_2$,...,$S_n$, the observed spectrum $S_{obsvd}$ would be the sum of $S_1$+ $S_2$+...+$S_n$. 

In FTIR you either measure the transmittance or the absorbance. The term intensity is more or less used when you deal with molecular or atomic emission of light (fluorescence etc.) As long as there is no chemical reaction among the constituents, the absorbance should be linear with the concentration.