Apologies, I don't know how to do the math graphics so I'll try it in words
You could just solve your first equation for $T$....
Simplistically:
For a circular orbit orbital velocity is constant at $$\sqrt{(G/r)(M+m)}$$
So for orbital period: you know $r$ (also constant), so calculate the length of the orbit (circumference, assuming it's a circle) and period is just length / velocity
Notice for a given primary the only thing that really matters is $r$.
Mass of the satellite (assuming it's artificial) is kinda negligible ($M+m$) where $M$ is the primary.
You are right about the geosynchronous orbit...but the period is determined by the radius...
props to HDE 22686 for adding the math graphics.