will mycelium grow if put in a liquid grwth medium like malt or dextrose like spores will ?
Yes, the mycelium will grow in such a liquid. Several growers do use liquid growth mediums, mostly as a clean innoculant of a cloned strain. Growth can be very rapid when you inject jars with such a liquid. There are a few posts about it on the shroomery.
In my experiments with it, I found it contaminates quite easily. I have heard of some antibiotics being added to prevent that, not quite sure about how that works.
IMHO, it's a waste of time; you should go with agar instead, it is much more reliable and scientifically documented.
I think indiashroomer has a point, after all, its east to spot contamination growing on agar, but it may not be so obvious in liquid culture. You could end up innoculating a lot of substrate with contaminated innoculum.