I had problems when I tried growing tomatoes in a container mix that contained coir, and also had problems trying to grow orchids in coconut fiber chips several years ago. The problem seems to have to do partly with the wide variability in quality of different products. Coconut fiber needs to be soaked and drained repeatedly to leach out naturally high sodium content. Some manufacturers do this before putting the product on the market. Others apparently do not. If you google "coir seedling mix problem" you will find many discussions of the problems people have had.

I would not use a coir product that wasn't specifically formulated for sprouting seeds without at least trying to leach out any sodium or other contaminants. And I would not use it even that way as more than half of your mix. Milled sphagnum moss (not peat moss) is the old standby for seedling mixes because it contains natural antifungal and antibacterial properties.

Thanks, I think you are right. I am not going to start that many seeds this year (I keep telling myself despite the catalogues with their beautiful pictures) so I am just going to buy something designated for seeds. I'll look for the spaghnum moss you mentioned, thanks.