I have the Fluval 306 on a 40-gal and it's considered by some to be overkill. I love this filter b/c it's very easy to use. I bought a 206 for a 20-gal I'm going to set up. So in answer to your question re: an extra HOB, I really don't think you need it.Indecision, indecision!
I went to Petco last night and left empty-handed because I couldn't decide between the 20l, 29g, and 40b in the ten minutes before the store closed. I like the proportions of the 20l and 40b, but I feel I'd want something bigger than 20l. And the 40b would add a bit of cost due to needing the next size up in lighting fixture, more substrate, and possibly a supplementary HOB filter. Plus, we're in a rental, and I'd hate to have to lug a larger tank around until we finally settle.
I'm finding I like tanks that look like they are a "miniature scale" scene, but it doesn't seem like I'd have enough luck to do a carpet without C02 to pull of an iwagumi style. If I find tall pieces of driftwood and attach moss/plants to the branches, is it possible to make good use of the vertical space of a 29g and still create a "miniature" world?
I am also now torn between the Fluval 306 and Eheim Classic 2217. Both seem very comparable. I was initially going for the Fluval because it was $20 cheaper, seemed more beginner/user-friendly, and the media baskets sound easier to maintain. But since Hydor's in-line heaters come in 1/2" or 5/8", I'd have to buy new hosing, clamps, and rubber adapters to fit it in-line with the 306's 3/4" hosing measurements. Those additional parts would make the set-up between the 306 and 2217 about the same price.
How do you people ever decide on anything?!
I can guarantee you that no matter what size you get -- 20 gal, 200 gal, 500 gallon.... you will always want something bigger. ALWAYS. So that being said, I would say just get what you can comfortably afford. Also, something that a lot of people tend to neglect is the number of electrical receptacles they have. Every time you add a heater, add a set of lights, add a pump/airstone, etc... that's another plug. Make sure you have space for your setup, and have enough plugs in close proximity. And using multiple power strips can be dangerous, so try not to plan to use more than one in an outlet. I'm sure even that has got its problems.
Oh, and not sure about the heater -- do you really need 200w on a 29-gal?
Guppies -- if you really really really like guppies and want to figure out what to do with 20 babies every few weeks -- then go for it. I put 4 guppies in my tank (1 male, 3 females) and a week later had nearly 20 babies swimming around. I bought 2 pearl gouramis and they have gotten the baby population down to about 5 or 6. I'm not interested in doubling my guppy population every month and having to figure out what to do with them so hopefully the gouramis will keep eating them!