You can only have one active at a time.
Do you even know what a rom is GraphicDemon?
The Droid can easily be customized without rooting or flashing a new rom to the device. Most of the features you find on roms you can piece together yourself and use separately. By reading your questions it seems like you don't know what a rom is and you have never owned a smart phone in your life.
You could always sift through a bunch of ROMs that have things you like, and then use Relentless ROM Roaster and put the parts you like all into one ROM. I think that's what RRR is for. I wouldn't know for sure, since I've never used it nor have I really looked into it... but from what I've (briefly) read, I am pretty sure you can use it that way.