I Have A Fixie With 700c x 25c wheels. Will I Be Able To Put 700c x 38c tubes and tires on my fixie rims?

You probably can, but it will depend on the internal width of the rims.

Look at the attached link, and scroll down to the bottom.

you'll see a chart of acceptable tyre sizes for different rim widths, although there is a disclaimer saying that many riders do manage to go outside the recommendations without problems.

I've gone the other way, I have a performance hybrid that originally had 35 mm tyres on it, and I replaced them with 25 mm ones.

I have 17 mm rims, so I'm just inside the limits of the chart.

EDIT:- If 38's are going to be too wide, you could compromise by fitting 28's or 32's, they will be bi ger than your current 25's, but it will depend on your reasons for changing.

No, sorry. If it is a 700c standard wheel, then you can only fit a tire that is 23c - 32c.