[Is this wood safe for my fish?]

I think it would be fine if you boiled it good. As for the salt that is not a problem many fish keepers put salt in their aquariums to treat disease or when feeding brine shrimp. 

[Wagtail Platy Fry]

I think that they should be fine I have had platy fry in a lightly planted 20 gal. It had small 2 Amazon swords a 9 inch driftwood piece and a rock with a school of black skirt tetra and bloodfin tetra only two made it I had only ever seen 3 or 4 fry in there if you want to be safe do a breeder box.