If i bought an amp that is like 800-1000 watts and i only use about 500-600, would the fuse rate drop? So basically i don't want to upgrade my alternator or anything like that right now. But i do want to invest on an amp that i could use on my future systems.
You should be fine. If turning down the volume isn't an option then don't use your accessories. That could be uncomfortable and a bit dangerous on a hot, rainy night however.
Also remember that stock alternators provide very little output at idle so while waiting at a stoplight or demoing the system in a parking lot, etc. make sure you rev up the engine a bit while cranking the tunes.