Sorry 'bout not having time to address you all, but i just had to write this down. WarMedic, you're actually pretty close to the "powerup" system i've envisioned during this vacation. What i'm thinking is this: Instead of collecting ammo for individual special weapons, you collect "supercharges", which, when used, will fire off a massively powerful version of your current weapon. For instance, if you had a beam weapon, the supercharge would make it fire a HUGE beam that does crazy damage to everything in a line. Other powerups like those you mentioned aren't guaranteed to be included, as many of them can be bought in the shop. Things like double/triple shot, shields and maybe regeneration will be available for a price, though these will obviously be very expensive.
As for the getting of supercharges, they can either be dropped randomly, or from randomly selected foes that will be supercharged themselves. Supercharged enemies would be really tough and dangerous, but the reward of killing them is well worth it, namely a supercharge plus a fuckload of points, cash and XP.
That's all for now. I'm still in Spain so i can't really do anything, even if i wanted to. :U