You should take protein and creatine as your key supplements or a Weight Gainer product if you want to gain maximum mass. Take a whey protein shake 2 to 3 times per day with one serving being just before your Halo immediately after training and take creatine monohydrate powder or a creatine tablet (like Creakic or Creadex). Start off taking 4 servings per day for the first 5 days and thereafter take 1 to 2 servings per day (2 on training days and 1 on non-training days). To gain mass at the fastest rate, you can either do the Nitro/Cell stack (Nitro 3 times per day and Cell-Tech twice on training days and once on non-training days - always take one serving of Nitro immediately, followed by Cell-Tech right after training) OR you can use a weight gainer like Mutant Mass (along with powdered creatine) or Mass-Tech or Muscle Fuel Anabolic (which already contain creatine). If your diet is good (high in calories and protein), go with the Nitro/Cell stack, if your diet isn't so good, go with one of the mass gainers - start off with 2 to 3 scoops twice daily and increase until you are gaining the desired amount of weight per week (1/2 to 2 kilos).
Many advanced users stack several compounds in order to get maximum results from their cycle. Normally when your stacking these types of supplements its best to use single compounds instead of premixed stacks. The problem with using a premixed stack is you may get more of one compound than you want. Most of these products have sweet spots. Lets use h-drol for an example according to many of the forums out there 75mg is the sweet spot for this compound. Now lets say you have a halodrol product that is mixed with 10mg of Methylstenbolone. In order to get the proper amound of hdrol you will get more than the recommended dose of M-sten.