Yeah you're right FR tables that are available are so much limited to 6max and when you do find a FR table - it's usually full and people waiting. As for 6max, the difference is quite huge - especially the swings. So if you can't handle variance then I'd stick to FR and go to a bigger site, maybe pokerstars
. It is a lot more fun though 6max and there does seem to be more fish but you will often find in most hand you are involved in if there has been betting pre flop that you have to be willing to stack quite often. The all-ins and aggresion at 6max are so so much more than FR. I play both, although I'm grinding out FR at the minute as I took a hit at 6max and my BR dwindled quite a lot. I think I lost about 12 buy-ins in one session.
FR gives you the option of playing tighter and the swings aren't any where near as bad like I say. If you get rake though or some sort of bonuses for fpp's then 6max offers this so much more than FR so it's a bit of both really, you can make a lot more in rake but also take a big loss in variance.
As for shortstacking, I don't recommend this - not at 6 max anyway. You need to be fully stacked IMO to take advantage of all the loose calls and you will often be shipped on a hell of a lot more with a shortstack. If you do plan on playing short stacked though, then you're going to have to be extra aggressive, ship it pre-flop or post flop or again you're going to be finding yourself constantly topping up and losing money. Be prepared to playing looser as you can't play tight at 6max as the blinds will just cripple you.