Your planned setup sounds good. I have two of the Aquaclear 70's on my 55 gal goldfish tank. You just might need to buffer the flow because even on the low setting they have a tendency to blow the sedentary fancies around.
Substrate is tricky. You could try the basic gravel you get from petsmart or not. There have been cases of goldfish swallowing the gravel and getting it stuck. I chose to go with large river rock like you would get at the dollar store or some place like it. The other option is sand, but you have to worry about anoxic pockets.
For plants, it's hit or miss with goldfish. Everyone seems to have luck with different things. Some people have luck with anubias, some with swords. I'm having luck with hornwort as it seems too prickly for my goldfish's tastes. One note about having the large gravel, plants don't grow well in loose gravel, so it's best to put the plants in little terracotta pots. That's how I'm growing my anarcharis, and it seems to do very well. If you do go with plants, you're also going to have to make sure you have the right lights. Too little or too much and you'll get a ton of algae.
But for right now while you still have those little guys in a 5 gal tank, you're looking at daily 100% water changes just to prevent the ammonia from getting high. Cycling it is going to be nearly impossible.