If you have room for a 6" wide filter, you could get
the whisper 10. The one thing to make sure of though is that it has a flow control on it. Take a look at the photo in the link above, and on the top of the filter there is a circular hole on the right. That is where the flow control is (I know the 20 has one but I'm not sure about the 10). If it does then it would work fine, and yeah you'd have to turn it way down because this filter is good up to a 10 Gal. But the flow control makes this super easy and then you also have a good filter if you ever want to move it to a larger tank. There are other filters that are rated for smaller tanks like the
Aqua Clear Mini 20 that might work too, but I've never used one so I can't say if it works well or not. As for the heater, it shouldn't be a problem. Just place it next to the outflow of the filter and it won't build up enough heat next to the acrylic to be a problem. Have fun! (and yes I do stroll around the pet store just for the fun of it :

)