Hi,
Working in a gym, the question of supplements constantly comes up. Protein supplements are alright if you're looking to put on weight, however the type of training that seems to be best for PRMC are bodyweight exercises. For these, the type of strength needed isn't absolute strength (as in 'I can bench press more than you') but strength to bodyweight ratio. There are people who are massive and the larger a muscle is typically (but not always) the more it can lift. But, while larger muscles can increase absolute strength, it doesn't always increase strength to bodyweight ratio.
For my training, I'm not taking protein supplements because I'm getting enough protein in my diet as it is. You might want to take them if you don't get enough protein in your diet (if you're veggie...). The only supplement that will help with strength increase is called Creatine, BUT I wouldn't take it. Creatine allows you to create more energy in a session, letting you do just a few more press ups. There are downsides, not the least of which is that once you're in training, I assume you can't take creatine, and the effects wear off shortly after coming off it. There are also other possible side-effects that aren't all that well documented. I've known people with some indigestional problems from it.
Simply put; protein supplements work for specific types of training, usually anaerobic muscle mass increase. From what I know of RM training, it's more aerobic muscle endurance, which involves a different type of training.
If I'm wrong on any of this, which is possible, it'd be great to know!
Cheers,
Tom