More stuff on the bottom just makes it harder to vacuum. But as long as you're ok with that, and as long as the fish have enough room in the tank, it should be OK. Do you have bottom dwellers? If so they need adequate bottom space to hang out in.
Should be pretty easy for you to move the fake stuff around and then vacuum. My problem is that I've got heavily planted tanks... with real plants that have rooted into the substrate. This makes it pretty tricky to vacuum around all the plants.