The systems work just fine together. It is just a matter of what you want them to do size wise. I have 2 closed loop geothermal systems heating my house, one water to water and one water to air for a total of 9 tons. I added a 50x60x16 shed to my heat load so my geothermal systems were not big enough then. I purchased a G series Heatmaster to handle all heat loads and DHW, now my geothermal is my backup. About 2 months after ordering my OWB I found a Geothermal system on Craig's List with all components (same make as my home's) big enough for my shed for $2200 including an air handler for A/C if I want. It was one year old, the guy didn't like it saying it was too loud. When I'm sick of cutting wood I'll put in a loop field and put my shed on geothermal too.
The point to all this is that you can make both systems to work together. Slim was fantastic in aiding a newbie like me at the time to the OWB world. The controls were not that complicated to configure as I had extensive experience in HVAC and Slim helped me figure out the best method for tying in the OWB. If it were me, I would put in Geothermal big enough for anticipated heat and cooling loads for the day when firing up that OWB gets old or your body says enough. Just my 2 cents worth.