#1 Figure out what you want to do with your body

You want a six pack but dont want to lose weight, sorry but thats a contradiction in terms... to show your abs you need to lower the body fat % down to around 8% for the whole 6 pack. You cannot dictate where you lose the layers of fat from (enter the realms of genetic tendancies) and its a case of playing with the hand you were dealt.

If your thinking that your too skinny then you needed to of added some muscle size before you went on a 'cut'

Figure out what you want to do, you cannot lose bodyfat and gain size at the same time...it is feasible but nigh on impossible for someone of your training experiance (from what i've gathered from your post).

If you want to put mass on, put it on evenly, training just your upper body looks retarded and i'm not joking when i say that people will laugh at you when you put on shorts

To stop yourself losing weight you need to up the calories above the level that your burning, i'm not saying to go nuts... raise your calories by 200 each week and after 2 weeks monitor your look in the mirror rather than on the scales. Weighing scales does not differentiate between muscle gain and fat gain

Supplements? In you case....food, from quality sources and no junk