Different kinds of questions

[Wanderer]

If that is the case I build first couple of shield-platforms and then weapon-platforms. What if I have cargo base in planets orbit and I move all weapon platforms there and then put them back so I can put shield platforms first and weapon platforms then and the sensors platform Last. Does this work?

No. Units on a planet are destroyed in order of hit-points, lowest first*. That's why troops/mines etc. get wiped out before the weapon platforms. Your 'shield' platform will be the Last one to explode I'm afraid.

I used to use the method you've described, until I realised it wasn't working.

*Unless I'm very much mistaken, of course. It's been discussed on here recently, IIRC.