Wing Tank Pumps & CTR TK pumps relation with Slats
Ahmed, what you asked is a very good question. The Design logic in A320 is that the fuel is fed to engines from CTR tank first then the Wing tanks but when Slats are down, it is normally Take Off Config and we don't want any problem in fuel feed to engines during this time in case CTR tank pumps block/fail and so Wing tank pumps run with slats down because Gravity feed is possible from Wing Tank pumps should they fail.
And irrespective of slat position during engine start the CTR tank pumps runs for 2 mins because it is just to clear out the Water in the fuel tank bottom before the a/c enters the crucial phases, Actually burning away the water with fuel in the very beginning....
Just thought of sharing my knowledge though its too long since you asked this but may help other brothers... Peace....