I have lately had people ask me for advice on if a particular set up will work and in a few cases it is obvious that they have done some research but have mixed their methods and are using the stocking/feeding rates for one method but trying to apply them to a totally different situation.
A combo media/raft system which retains it's solids in the media beds should be using the recommended stocking/feeding rates for media systems and low density raft culture with no solids removal.
However, if that system were to be stocked as if it is a UVI raft system and fed at such rates, the gravel beds are going to be overwhelmed with solids and the bio-filters will not be able to keep up. (Perhaps this is where the opinion that gravel beds clog and need cleaning comes from.)
It is important to remember that solids removal is a very important part of the UVI model and if you want to stock their numbers of fish, you had better design according to their model.
There have been newer developments into lower density systems that don't require solids removal but it is important to note that they are lower density systems. That means less fish and less fish feed to get the same amount of plant growth while not wasting the solids by removing them from the system.
Now what kind of system you design will depend greatly on your goals for the system. The lower density media and raft systems will provide more plants with less fish feed costs and of course not provide as much fish but probably also be lower tech/lower cost to start up and require less filter cleaning. The higher density solids removing systems can provide more fish for the tank/system space but there will be added costs in fish feed/aeration/maintenance and the solids become waste to remove from the system (this might be great if you love to use the fish waste in your dirt garden but a problem if you don't have any handy soil garden or compost to feed.)
One should also remember that though a raft system or an extra fish tank and a huge amount of extra water in a system might prove beneficial, it is not the answer to all ills. Just because your entire system might hold 50 gallons for each fish in your fish tank, if there is only 1 gallon of water for each fish in the tank and your fish are getting big, then there won't be much dissolved oxygen available to go around in that crowded tank and any little mishap could be killing fish quickly. I rather like 5 gallons of fish tank per fish and when you start growing them out to larger sizes 10 or more gallons per fish is even better. Even when I only had 18 fish in my big system, I still had 20-40 ppm of nitrates which is way more than enough (I know people who barely ever register nitrates and their plants are still happy.)
A gravel bed system will provide long term mineralization of the solids once the system is mature and can run for long periods with minimal fish load and still keep plants happy.
When a system is new and the bio-filter not yet established, more fish is only likely to cause more dead fish. Get the system cycled up with a small load, then add more fish if you want them up to the recommended stocking levels for your system method. After that, if you think you need more, expand the filtration before you add the fish. Trying to play catch up by adding more beds as the fish grow is not the right way around cause you will always be playing catch up. Tip, having too many fish because you are impatient and want them to grow won't really work well when you have to restrict feeding to keep water quality under control. It would be better to have a small quantity of fish that can be fed appropriately for best growth so you have some fish earlier rather than a large number of stunted fish due to feed with-holding to keep the water quality good while you scramble to build more filtration to handle the too big fish load of stunted fish.