Hey, you are back! After so many years haha. What happened to your 2ft?
A pico tank is hard to keep. It needs lots of maintenance, no matter what kind of equipment you use, the space is an issue.
How small of a tank are you looking at?
The most challenging problem for me is the temperature. It wouldn't be a problem if the tank is situated in an air conditioned room. For a pico tank, I think an iceprobe would do the job. But then again you have to state the size of the tank first, before more advice can be given.