When I made my musclesuit, I did it in two pieces. There was bottom half that pulled on like a pair of pants and a top half that had a thong strap that went between the legs and snapped closed. The seam between them was covered by the loincloth and armor.
After four days of wearing it around I really wish I had made the arms separate. With the new style of armor that was in AVP, you would still be able to hide the seam at the shoulder and making it separate gives you much greater range of motion.
If I had it to do over again, it would still be two pieces. The first would be a full bodysuit with the arms and legs dressed to look like the predator. The second would be a torso that would zip on (you can hide a back zipper under the armor too) and cover the trunk.
I have a lot more about how I made my musclesuit and the rest of my predator on the Hunter's Lair:
A friend of mine was building a predator costume at the same time (only running about five weeks behind me) and was able to benefit from my advice. If I can dig up a picture of his suit, I'll post it for you.
Here's a couple of pictures that make the detached arm argument.
The first shows the two pieces of the musclesuit for my costume:
The second shows the whole thing put together. The first time I test fit the whole costume I actually tore the armpit while checking mobility. I was able to patch it and reinforce it and make the patch seamless, but the continuing problem is if I raise my arms above shoulder height, I also run the risk of unsnapping the thong strap that holds the legs up and the upper body down. And even the Predator can't look cool trying to button his crotch back together.
This is a picture of a buddy's predator. The finish work isn't quite as good, but the arms on this one are separate which allows for a greater range of motion.
This is a shot of both of them. Notice how AVP inspired shoulder armor on his covers the seam between the arm and torso:
I have painted up a few latex pred suits with seperate arms.Although there is much more freedom of movement you are left with gaps when raising the arms.
Usually they wear a black t-shirt underneath the costume but I think it would be better to spray up a t-shirt with the same colours as the pred.