Essentially there are two modes of collisions.
1) Fast collisions along the lines of what we have now, able to run on all platforms
2) SPH "collisions" which initially will only run on opencl supported hardware
SPH is implemented, but not yet quite hooked up in the build.
For the really complex things such as a gas cloud collapsing under its own gravity, compressing into a central mass and accretion disk, which in turn lumps together futher, we will be going the SPH way. Same goes for things as the moon forming impact
SPH Example - Not Universe Sandbox and not running in real time (it's pre-computed)
The formation of the Moon For the general fast collision system, there will be limits. We are however working hard to expand those limits as far as possible into the realm of awesome :-)