WIth the stock that you have I would very much be wanting to add to the Rio 300's own filtration. THe Juwel units are good, and I would not want to replace to it, but add to it.
THe Clear Seal filter you suggest is OK, but not ideal. If your budget can stretch to it I would really advise getting an external cannister filter that, firstly, will not take up extra room in your tank. Secondly you can put proper biological filtration media in it. The CLearseal is a foam and carbon only filter ( the carbon being fairly useless except in specific circumstances). Proper ceramic, sintered glass-based bio media (like Eheim Ehfisubstrat or substrat pro, Juwel Cirax or one of the many others available) has big advantages over foam-only filtraton as the media has a much higher surface area allowing much greater biological filtration.
If you can stretch to it, something like an Eheim 2215, or a Fluval 205 or a Tetratec 600 or 700 would add superb extra filtration to your tank. Some of those come with decent ceramic bio-media, some do not. But you can always upgrade your media at a later date (also as your fish grow, and add extra bioload onto the tank).