In Fireworks, I can hold shift to show the distance between two guides, is it possible to do so in Inkscape?
If not, what is the option left to position everything symmetrically?
You can double-click on a guide to see its properties. There you can specify the absolute position, or a relative displacement.
Enabling guide-snapping, you may for example drop a new guide onto an existing one, double click and then move it by a precise amount.
You also have Path to guides (shift-G), which converts an object into the boundary guides.
Extensions -> custom -> custom guides may also help.