Weight and gliding
If they can glide, but not fly, then that means there is an acceptable amount of weight that they could have on them and still glide with a fair amount of success. Of course flying is another discussion, as any weight added would have to be physically lifted together with your own body weight and that could easily make the difference between a steady climb and a steady fall.
I would say that they could probably carry at most 30% of their body weight when they glide. Just speculating, but my guess is that they would be very attached to their ability to glide and would use it whenever possible. As such, they would likely not want to put on any more weight than they'd have to. While they might be able to wear light armor, it would easily approach that 30%, and they would likely prefer instead to go without the armor and glide farther, even in battle.
Leather armor might be a far more practical solution for them. It likely wouldn't cover their wing whatsoever, but you might see many of this species with a sort of leather vest with holes arbitrarily added for their wing arm to pass through. My guess is that these holes wouldn't have been added by the original creator of the vest, since it is not likely that this species would master the practice of creating leather vests if humans (presumably if there are humans in this world) would have already mastered it. It would be easier to simply buy one at a human town and tear a couple holes in it.
However, this also depends on:
- if they are adept at bartering (they may not be particularly bright creatures) and
- if this species is tolerated in human towns (albeit as second-class citizens but customers nonetheless).
Also, a likely weapon of choice would be the spear or lance. It works as a close-range weapon and works to their advantage when thrown while gliding. It is also aerodynamic and therefore not encumbrant to this species while gliding. A more ranged attacker would likely prefer javelin to the spear, having more of them.
A possible close-range weapon for this species might be claws/fist weapons. It is light-weight and it would be easy to approach your enemy from behind.
That said, remember that there will likely be exceptions to these. Weapons that you will likely not see such a species use for self-evident reasons:
- Battle axe (too heavy)
- Nunchucks (short-range, impractical with respect to say claws/fist weapons)
- Long sword (too heavy, short sword maybe)
- Bow of any kind (encumbrant and perhaps even a little difficult for one with so many limbs to pull back without geting in the way)
- Shields (too heavy, would likely interfere with gliding)
There tends to be three types of diet for a bird, and you don't often see a bird with one diet cross over to the other kind of diet, though there are exceptions.
- One type is what you described. Seeds, berries, nuts. Generally whatever they have the fortune to scavenge.
- Another type is vermin such as worms, catepillers, maggots.
- The last type, you generally see with larger birds, generally fish and creatures easily caught while flying (or in this case gliding) over water.
If this species were advanced enough agriculturally, and they had a diet of seeds, berries, and nuts, you'd likely see many berry bushes grown near their village. If the species ate fish, then you'd see many "fishermen" that catch the fish and sell them in the village for money. For the diet of vermin, you can't generally grow that, but it would be a source of food for this species in a party where other human members may not have anything to eat (might make for interesting rhetoric between a starving human and a satisfied bird creature having had his fill on vermin).
Hope that helps!