You don't say whether or not you live in the US, so the site recommendations are a bit difficult. If you are in the US, you are limited to stars, full tilt, bodog, WPX, tribeca, tain, UB, AP, and maybe a handful of primas depending on what state you live in. Stars and Ful Tilt are by far the biggest of those, and have the larger games more often. If you live outside of the US, I hear party games are juicy as ever, but I would not know. . .

You should not shortstack any level below the 200s IMO. Proper shortstack strategy requires an aggressive table, something that is not common below the 200s. In addition, a large number of the PLO games (at Full Tilt anyways) are 6-max, which is not conducive to a short stack strategy.

The play at the 50s is not remarkably different from the play at the 100s. Whichever you chose, I recommend the following strategy: Buy in full, don't bluff very often except for picking up orphans, and value bet like mad. I have used this as an effective winning strategy for the low limit PLO and PLO8 games.

Post trouble hands here and check equity on some random hands at www.twodimes.net. And Welcome to BTP!

There's no point playing shortstack in PLO. Online stacks are short anyway.

The sites that you should play at depends whether you're american or not. If not, Party and perhaps Cryptologic. If so your only choice is Stars really, though there's a few others with tables.

Party is still the best site pretty much.