The 3D model in GRID is just for visualization. The configuration of the mechanism can be found in the configuration package: config/mech/mech-parameters.xml.
There you can change the architecture of the robot and which solver to use. Look at the example file to see how this file is structured. Unfortunately we did not publish the file format specification yet but it should be reasonably straightforward from the example.
Your 3D model in GRID should then also be modified to represent the new mechanism configuration.
Be aware that depending on the IK solver that you specify, certain rules apply for the kinematic parameters. For instance the Anthropomorphic solver assumes that the last three axes coincide in the same point (wrist joint). This is the architecture of most industrial robots.
The Anthropomorphic2 solver is used for UniversalRobot-type robots, where there must be three parallel axes.
Then there are also solvers for planar. For general mechanisms we support iterative IK solvers (KDL_NRJL or IKFAST), but these are much slower and are not recomended.
Another limitation of our standard robot-app is that we support at most 6 DOF robots, but this will be added in the future.