Its all good brother. The two products are popular, not because of their effectiveness but mainly due to marketing. Cell-tech is a standard mixture of creatine and dextrose, which is a good combination post-workout. It does contain small traces of ala, which is also a good product. But you can make your own batch or go with other creatin transport product with the same effective, and most likely, for a cheaper price. The nitro-tech is (imo), one of the better qualtiy whey protein source. But with that said, you can supplement any other quality protein powder and bump up your protein intake/day. Im not gonna bash MuscleTech's products b/c they do a good job in manufacturing products and its good quality, but dont necessary buy into the over-hype, just because they market the nitro-cell tech combo in every magazine and promo you see out there. So to asnwer you question.... YES, you can sub in any other protein powder
Sage