-3/4 isnt too much. as long as you can shake it, the jar is fine. if you want faster colonization (but less substrate) then fill it less. i fill mine 3/4 w/ rye and it works great.
-if you are steeping (cooking the substrate at a low boil) then dont add any free standing water to the jars. the cooked substate (seed or grain) will have the right water ratio already. so no extra water to the jars unless you want a goopy mess.
-ive never used quinoa or birdseed. only rye grain.
-daniel.