Here is some of the missing weapons:
Needle Gun
Laser Sword
Well a Power/ Laser/ Monofilament sword should be covered by the "High Tech Combat Blade", the Save/-4 basically negates all Saves!
What exactly should the game effect of a Needle gun be?
But I think we all can easy do our own stats for it.
Yes! That is exactly the spirit!

Just to understand: how many figures per side are meant to be used? =)
Depends on the Tech Level you use, at low tech you could end up with 25-35 minis + some AFVs at 1500 points.
High Tech is totally different, 5-15 infantry and a VTOL or tank.
And how big should the table be at last?
I'm more of a skirmish man, so would 15-20 figs and one AFV per side work one smallish board, say 90x90cm? Talking 15mm here 
Some quotes from the book:
"You just agree on a point limit and both can plan their armies. The recommended Points Value is 1500 points per side. This will allow you to field a full platoon with command unit and some supporting units. It will give you a 2 hour game on the aver-age – depending on your playing style, how much you know the gaming table and your opponent etc.
The smallest game size recommended is 500 points. You may only field 2 Squads and very limited support, one mistake and your patrol is gone. The game will last between 30 minutes and 1 hour."
"It is recommended that the gaming table is 180cm (72” / 6’) on the long edge and 120cm (48” / 4’) on the short edge."
and
"In fact we found that the weapon ranges look more realistic when using individually based 15mm infantry miniatures. If you want to play with smaller tables, just use centimetres instead of inches and everything will be fine."
So the smallest table should be 50cm x 50cm using the Advance! scenario on page 58.