In my shop I mix components on a regular to customize a customer's gearing (Of course matching Shimano & Sram shifter/brake controls* with their respective rear derailleurs), which includes using road and mtb stuff in whatever combinations necessary. As has been said the issues with the front derailleur are the curve & length of the cage and the depth of the inner cage sometimes (Different between road & mtb). I explain that some combinations will result in a bit of degraded performance but the drivetrain will still shift just fine and provide more than acceptable performance (For example Ultegra shift/brake controls* with an XT rear derailleur and 34 T large cog).

*brifters needs to be banned from use, it being the most stupid of terms.

In the OP's case, the goal seems to be to use the least expensive rather than the best combination of components so performance isn't necessarily an issue. Therefore he should just throw whatever on the bike and then decide what does or doesn't work. In the 12 hours this has been going on, he could have done that.