First of all, so sorry for the late response on this, Bloks has gone a bit inactive and I feel like it still needs some work some time in the future (it just looks a bit unprofessional and is a bit clunky to use if I'm honest, but all the features that are shown work fine)
As for your questions:
- The grid isnt exported with the model, so the grid size doesn't really matter
- I'm afraid its not possible to import your own primitives.
Perhaps Asset Forge (by Kenny, its a fantastic tool) might be a better fit for you, that allows you to import your own primitives I believe and was generally what I was going for with Bloks (apart from the fact that bloks is in that first person, game like view).