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The actual size of your various pumps depends on several factors but probably the most significant is what level of circulation you want in your tank.
You obviously want good quality circulation (i.e., reaching all parts on tank in randomized flow patterns) but the magnitude of circulation will dependend alot on what type of critters you plan on keeping. If fish only, then more moderate flow levels are sufficient in most cases but if as I assume you plan on having corals then more substantial circulation levels are typically appropriate and if SPS corals are planned then you likely want even more figorous circulation.
Once you have determined the level of circulation you need then you have to determine how it will be distributed between internal (includes closed loop) and external systems. My normal recommendation is to use just as large a external system as needed to provide enough circulation to provide for temperature control from the sump (just a few hundered gallons/hr is normally sufficient) but depending on your systems requirements you may decide on a higher rate of return. The bulk of the circulation requirements can normally be handled by internal or closed loop systems more efficiently than the external due to lower head requirements. Use the procedures I outlined in my articles along with your specific installation requirements to determine if a given pump is properly sized but for your closed loop system you do not normally need pressure rate pumps.
As for redundant sump pumps for reliability - if you plan to always have someone home who can be there to detect a failed pump and be able to repair/replace it within 24 hrs (assumes closed loop pump(s) are still running) them maybe you do not need redundancy but I know few folks in this situation (most take vacations for a week or two and even if they have someone checking their tanks they may not be sufficiently knowledable to handle emergencies of this type). Also if you have a spare pump to replace a failed pump why not just have two pumps in system as it costs little more. Your concern with multipule returns to tank with redundant sump pumps I would normally consider an advantage but if your installation requirements limits the number of returns you can accommodate then you can combine the two pumps outputs into one larger return (you need to use check valves on both pumps before combining them to keep a failed pump from shunting water back into the sump).
RN
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