Shinigamis don't go around doing that because it's illegal in Soul Society...
Technically Aizen would be able to, but it would take a shitload of time... And we don't know if it will affect all people like it did to Ichigo... Probably if the same thing was done to someone else he would've ended up being a shitty ass shinigami, so why bother?
And to answer you question about gigais...
Gigais usually help shinigamis recover their spiritual power, they are made up with spiritrons (or whatever they are)... The gigai that was placed on rukia didn't posses this, rather it was made of something (unknown to us) that aborbed her spiritual power, trying to transform her into a human being...
From what I can see since she is in soul society, she didn't go through the transformation so she should still be capable of recovering her lost spiritual powers (I'm not sure about this though, I don't really understand what they are trying to say when they say "to make her human")...
And you are misunderstanding the point he was trying to make with demon arts, He was merely stating that her demon magic was so weak because the gigai was preventing her from recovering her spiritual powers...
@DarthEnder, there was something important that was ommited in the anime from the manga... This should've happened right after Rukia was taken to Soul Society...
Some investigators found and examined Rukia's gigai and commented on something along the lines of "Wow, I've never since a gigai like this before... Whomever did it must've been very talented..." Of course, you can tie up this with whatever we have just learned from Urahara in this episode...
I think someone else mentioned this earlier in this thread... maybe someone can post the pics to what I'm reffering to if I haven't done so already...