Willravel

Something like this would make a good jumping-off point:

Of course that's not going to be enough, but at least you get shoulder, elbow, and forearm protection. It might be good to combine a jacket like that with a used ballistic vest to protect your abdomen, chest, and back. You may also want to get better forearm protection because they're going to be so close to the action. So you'd have two pieces as the base, shoulder and arm armor and an armored vest.
I'm starting to think the best option for the armor itself is elastic bands and velcro. I'm more of a hand-to-hand combat kinda guy, but I have to imagine there's a ton of movement in melee combat. I wouldn't want heavy plates swinging around, throwing off momentum and balance, so it stands to reason you're going to want your armor as close to your body at all times. Motorcycle armor I've seen seems to follow this same logic. Check out these forearm and shin guards:

Those buckle right on to your body and are held tightly in place by several elastic straps. Imagine if, instead of cheap plastic, those were steel.

Maybe what you do is have the ballistic vest, with some shoulder protection on top of that, and then you have a series of pieces which you attach independently, everything from upper and lower arms to thighs and shins.
The gladuis is a one-handed weapon. Will you want two of them, or are you interested in also having a shield? Because having a shield will change things significantly.
I understand.

I'm thinking there are people in this thread who have experience with military body armor. I suspect their knowledge of this would be very, very helpful.