Tannin/softener combo units work, but the tannin levels must be very low. The Anion resin is easily fouled by and should be softened prior to going through the tannin resin. This poses a problem for the combo design since the anion resin will naturally seperate and rise to the top above the softener resin (cation).
I always recommend a res-up or similar resin cleaning program for these combo units, and they are not recommended for larger application (1-2 bath is my maximum recommended application, no mega showers or tubs).
For this design, the larger the better. I do not recommend a small system since the amount of resin is limited since you are sharing the tank with 2 different medias. We have several customers that are successful with this design, but they can be tricky. A fishy odor is also a common complaint when the systems are not set up correctly or if they are operated beyond their capacity.
A water softener producing 0 GPG may not feel soft. 0 gpg can range from 0-3 on many softness test strips. A gpg is 17.1 ppm, so you could be exceeding 50 ppm and the test strip may show you 0.
I highly recommend the HACH 5B test kit, it is not good for low level testing (<17.1 ppm), but to ensure you are below 1 GPG, it is highly accurate and inexpensive. Test strips are not a good or proper test kit except for verifying that the system is, or is not working.