I want to make my own mix for starting seeds. Can I use just Coir and worm castings, or should also add a light peat-based mix?
It's just one study, but you might want to read through this before committing to starting seeds in coir. I have no personal comparison to add, haven't used it for seeds -
Here is a link that might be useful: Coir vs Peat
Agree with the above - there are some issues associated with using straight coir. It works much better as a secondary component of the mix rather than the primary ingredients IMO. The quality of the coir used can also make a big difference.
In our GH we have done some informal comparisons between coir and peat based mixes and find a somewhat better level of germination with the peat mixes. Plus the peat based mixes (or a 2:1 peat to coir mix) have less fungus gnat issues and slightly better root development.
The finer the grind of coir the better the results it seems. We tied some bulk supplies of coarse ground coir and got very poor results from it.
Granted these are very unscientific results and there are lots of variables than can affect the results, but as you are going to have enough issues arise from using the worm castings that I'd encourage you to incorporate peat into your mix.