Quick suggestions.

1. To increase UI usability of Fleet Customization menu, please add highlighting of appropriate slots:

• filter is "All". When I'm moving cursor over a module or dragging a module to drop it in a slot, it will be good to see slots where it can be placed;
• filter is not "All". Highlighting of slots that can be used by current modules type will help, too.

As example of highlighting, on the image above I used additional circle. I know it's used for "symmetrical" slots so far, but...

2. ...what if, instead of circle, symmetrical slots will be indicated as something like this:

For me, "symmetrical" means "from both sides", not "around"  

3. How about hiding all icons by pressing Space etc? You know, to see new ship in its best without going into battle.

A pulsating outwards circle could also work IMHO.