Aquaclear 500 in a 33gal? ok or not?

While you can't really over-filter a tank, you can create too much turbidity. What kind of set up is this tank? Any live plants? Will your fish appreciate the extra water flow or be hampered by it? Do you have a fine substrate, like sand (which can be moved around by too much waterflow until you have a big sand dune)?

It really depends on your set-up, the kinds of fish you have, and your bio-load. For example, for a cichlid or goldfish tank, which is probably plantless, a AC 500 on a 33 gallon would pose very little problems. If you have a lot of fast moving fish like tetras, or active livebearers, I think an AC 500 turned down low would be okay. However, if you have a lot of slow moving fish, like angelfishes, heavy turbidity might not be the best thing for their comfort. The turbidity might be a bit much if you also include the Fluval 204.

However, if you plan on using the AC500, I would use some of the AC 200 media in the AC 500, just to jump start the biofiltration in your new filter. I would also consider whether the excessive turbidity would cause a lot of surface agitation which would drive off the CO2 which wouldn't be good for your live plants.

Okay, the problems I see witht he AC 500 are thus: If it is on high, you run the risk of over 100% turnover per hour. Normally I would not say risk, but with plants, you may be gassing off too much CO2, which the plants need. If you can adjust the flow of the 500, I'd say that would be the best way to go. HOB filters can and do work in planted tanks (see my sig for details) but they are not ideal. I really do not think your fish will have trouble with the turbidity factor, especially on a lower setting. Fish tend to get used to such things. Really, the species you have listed should do fine anyway in my opinion.