Having a single ammo type means ammo is easier to refill, but also easier to lose, so it might encourage a different playstyle. Especially if ammo refills are very common and the ammo cap is relatively low, you're encouraged to spend ammo and incorporate special weapons into your playstyle, rather than be encouraged to save them up for when they're really needed. I'm not a huge fan of how Portrait Of Ruin made enemies only drop money if your MP gauge was full, since it meant you'd more or less only obtain money by grinding or not use special moves at all, so be wary of mechanics like that.
I'd personally say the best approach would be having exactly two types of ammo, e.g. bullets and energy; this would also encourage the player to switch between two different weapons (you could even tweak the odds so ammo drops for whatever you aren't currently using are the most likely... and make sure switching between the two weapons is fast, so you don't need to open the menu every 5 seconds) and try out different ones.
