I work as an animal trainer, and getting kitty to use the toilet is just about the most difficult thing in all of training an animal, it just goes against every instinct they have,...it CAN be done, but as most of the other posters said, it's more for the owner, and much less for the cat, THAT FACT, and the fact that it will take months, maybe even years to get kitty to go in the toilet is why it's better to seek alternate solutions (see below)
I also live in a VERY small studio apt, and have 2 cats, and one outside stray that stays inside when it's very cold, or raining.
my solution was twofold:
1)
odor control: no cat litter in the known universe will control odors effectively with 2/3 cats and 400 sq/ft of studio apt space.
solution: so I ran a dryer duct tube from the bathroom vent fan to a hole I cut in the (covered privacy) cat box lid, up the wall and across the ceiling, and just leave the vent fan on all the time.
now because the air flows into the catbox, and out the vent fan (ejects out the roof), I NEVER smell anything in the apt, unless one of kitties misses the box or something, and that's how we know, because you CAN smell something...... "honey?!, did one of the kitties miss the box? I smell something"
cost was about $5 (ducting, tape, plastic fitting@home depot)
2)
clumping cat litter stuck to kittys paws like GLUE!:
kitties always like to play in the shower after we get done using it (they lick the floor?
), then of course, they run into the catbox with wet feet, and the litter clumps around and in their paws.
solution: I made a clear plastic cover for the shower door: slips over the bottom lip, and velcros at the top, so kitties can't play in the shower, we just take it off and set aside when we take a shower.
CATS DO NOT LIKE CLEAR PLASTIC, it confuses them, they have trouble seeing it, and their claws don't get any grip, so they usually avoid trying to climb barriers made from clear acrylic or polycarbonate plastic, and it is what I now make all my "cat barriers" out of.
if you have your heart set on training kitty to use the toilet, you need to start with the cat box ON the toilet, once kitty gets used to that, then use a small bucket of litter IN the toilet (you have to drain the water after you use it, and set up the bucket), and use some kind of toilet seat that kitty can grip with their claws (they don't like hard slippery surfaces)
once kitty is going in the bucket consistently, just take the bucket away, and again BE PATIENT, as kitty will know WHERE you want them to go, but will be confused with no litter all of a sudden,....remembering that kitties learn by example, from mommy cat, SHOW kitty when YOU go, let them sniff around so they associate the toilet/poop, and what you are asking of them, and it'll take many, many tries before they get what you want them to do, be prepared for LOTS of "accidents" as kitty will get confused with this at first, keep at it, and eventually kitty WILL use the toilet, just please don't ask me how to train them to flush, I haven't worked that out yet
this will be TOTALLY cat dependent,..I know one of my kitties (ruk=boy) could learn this in a couple of weeks, he's just very compliant and easy to teach, and I KNOW my other tomcat (kyu) would NEVER do this, he's just too stubborn.
good luck, your going to need it.