I bought a small piece of live rock for the corraline that was growing on it, since my tank will be empty for a while, I decided I would go ahead and get corraline growing in it. The amphipods were just an added bonus. I am also working on a refugium and will get the macro algaes at that point. I was surprised actually to see these amphipods swimming around, there are dozens of them if not hundreds. Some are swimming some are crawling on the sand.
What you said makes sense, that is what I always did with my freshwater, bought lots of plants, then once they were established started adding livestock. Either way I plan on going slowly once I start adding critters. But that will be a while down the road, my rock that I made is curing, but it will be a while yet before it is ready. Until then I will happily watch my little amphipods swim, and hopefully get some good corraline algae growth.