I use a soda bottle method. I use a 2 litre soda bottle and cut a hole large enough for the small one to swim in and out of. Then I place that as close to the larger one as possible. The smaller one will swim out when it figures out where the hole is and it feels comfortable. You should also be feeding the larger one a tad bit more than the smaller one to allow for the size differential to increase. This helps eliminate aggression in pairing. Of course, you always have the chance the darn thing will kill it. My advice would be to capture the larger one and put it in the critter thing for a little bit. Let the smaller one find some spots to hide.
Just a few ideas. All of them have worked for me in the past.
