I pretty much was in the same situation you are in - only copper & silver and flux. You can slowly equip your soldiers with steel armour if you keep trading and melting every piece of iron/steel item you can find. Traded bronze armor works well as a substitute in the meantime and is quite cheap.
For starters, just train your armorer with copper, slowly building up towards iron or steel. Eventually, you will probably make some adamantine armor as well.
And bolts: yes, copper/silver is good. Even bone bolts can penetrate gobbos copper armor - but you'll drown in tetrahedrite and galena anyway, I bet.
The only real danger in gobbos are their ranged fighters. An elite gobo crossbowman ignoring the fortifications will usually kill at least one of your marksdwarves, or more if you cannot focus your fire on them (which is highly unlikely). Drawbridges built infront of your fortifications can serve as lever activated cover if your crossbowmen get under too much fire. Like so:
If you're really
concerned about keeping wounded marksdwarves alive, you can even raise the bridges, recover the wounded and resume fighting afterwards. It also has the advantage of not letting your dwarves see the enemy which can be useful if you want to reposition them, etc.
To the left of the picture you can see 2 ramps. While their ranged fighters are busy fighting my ranged fighters, melee gobbos will keep moving on towards the ramps. Beneath the ramp I station my axe / spear (yes, spears - I had an artifact weapon
) dwarves well out of line of sight of the ramps themselves (to keep them from heading towards the enemy too early. This way I do not have to worry about my close combat fighters being pelted by arrows.
Of course, you could also just build a google of traps... but you are playing DF for Fun, right?