....we're all getting into very random side conversations that are all very interesting, and I'd love to chat further on them.... however, I'm starting to feel like we're starting to ignore the original poster.
SDhky, I'm a big, big fan of... REALLY, super, incredibly, uber small tanks. So, I understand you on the urge to put big things in small tanks. It's mighty tempting. But, smaller tanks do tend to fluctuate harder with water quality than larger tanks. Now, I'm not going to argue the nitrates issue that other people have been discussing, but I would like to point out that salinity can take some pretty severe swings on hot days from evaporation in smaller tanks. While other people have pointed out the nitrates and other pollutants issues, I just wanted to give a quick reminder or something which you probably are already aware of with nano tanks.
Still, that's not truly the reason I would hesitate putting an anemone in a nano to pico sized tank. I would hesitate due to the growth size of the anemones and my love of corals. Anemones have a tendency to sting, damage, and kill corals. The larger the anemone gets, the easier it is for it to destroy nearby corals, especially more delicate specimens. So, if you like corals as much as I do, I wouldn't really recommend it.
If it is a question of hosting, there are several types of corals with clownfish will accept as substitute hosts in place of an anemone. Euphyllia (torch coral, hammer, frogspawn, grape, etc) are commonly adopted by clownfish as their hosts. Leathers (especially toadstool leathers with long polyps) are also good substitutes, but some clownfish will also go for Xenia species, mushrooms, Heliofungia (long polyped plate corals), large feather dusters, fake plants, and even clam mantles for their hosts. Some species of clowns are very easy to get to accept a host, even if it's not truly an anemone. However, I must also point out that tank-raised clownfish are less likely to exhibit the behavior of using an anemone or anything similar as a would-be host. So, if this is your end goal, consider all aspects to anemones.
However, if anemones are still the goal, might I recommend the condylactis, the green (or common) bubble tip, or the long-tentacle. These three are all quite hardy anemones. The condylactis is known for being low-medium light tolerant so long as it is regularly fed, but the condylactis anemone rarely hosts fish. When it does host, it takes a long, long time. The green (or common) bubble tip anemone, however, is one of the most common anemones used for its hosting of fish. It has a unique look, especially when the polyps swell, and, since they split so well, you can usually find some in local reef clubs for a steal of a price. The long tentacle is also pretty hardy, and hosts well. However, like I said before, all of these anemones will sting and harm corals. That, and don't forget about feeding it meaty bits of food (like small pieces of fish, mysis, squid, or krill).
Consider your options carefully before you commit to an anemone!
airinhere, to clarify, plate corals, neither Heliofungia or Fungia species, are not known for being extremely hardy. They are known for being easily beaten during shipping and ripping by inverts or torrential current. In addition to that, once damaged, the health of plate corals quickly goes into decline, and many reefers have noted that it can be difficult to bring them back to peak condition. All things considered, this makes plate corals rank around mid-grade of coral hardiness. As in, they're no where near as easy as leathers or mushrooms, but they're still no where near as hard as carnation coral.
This means, I would take it as a great sign that plate corals do well for you! It means you know your stuff about taking care of them, or you keep a well set up tank! Kudos!