I was running one of the Zoo Med 511s on a 10 gallon for a while. I was not impressed and ended up replacing it. The flexible tubes gunked up pretty quickly and needed frequent cleaning. It came with too rigid tubes for the intake and spray bar that were exactly the same...so the intake tube had holes that had to be plugged to keep baby shrimp out of them. I opted to not use the spray bar and the elbow pipe did not fit properly and kept falling off. The HOB filter that was on the tank at the same time always seemed to trap considerably more debris which leads me to believe that it was working better. The 511 might be ok for a smaller tank. I don't think I would bother with the 501. Keep in mind that the Zoo Med needs to be placed at the same level as the tank...so next to it or behind it...not below it. The Hydor heater is pretty large. I am not sure, placement-wise, if that would work out very well.
I absolutely love my eheim filters. And I think, if you are going to look at the larger ZooMed, then you are probably in the same price range as the Eheim 2211.
I have not yet had to deal with flow reduction on my Eheims as I have them on larger tanks where it is not as much of an issue. But if you dig around, I think you will find that there are various options for controlling the flow. I don't think putting it on the floor is going to make much impact. The eheims are designed to run below the tank.
I have Hydors on my larger tanks also and I really love them. They are consistent and I have had no issues. There is an audible click when the heater turns on and off that may be an issue for you. I've gotten used to it and it doesn't bother me. I don't know of a smaller external heater. All of my small tanks have internal heaters.