[Freemananana]

Use the 60 gallon "sump" as your main tank and convert the 15 to a sump. It's easier to maintain one big tank than a whole bunch of smaller tanks. Less cleaning, more stable, etc. IMO you are asking for a plumbing nightmare to try and hook these tanks to a single system. What's the benefit? Lots of risk overflowing the tanks, disease, algae, etc. affecting all tanks.

I think the 60 gallons was for total tank volume. For me, sump size is just a physical need to work inside of. The bio and mechanical demands for freshwater is low. You could use a 10g tank for a sumo easily. Some foam and filter cloth for the mechanical and pumice for the bio needs. A cheap drawer system if you want a wet dry or just the tank for a typical submerged sump.

Unless you are overflowing from one tank to the next to the next to your sump, 3 pumps and overflows is a lot. The diy PVC overflows to replace boxes don't work that well IMO. A nice air pump and sponges seems more reliable to be honest.