Yeah..6' diameter is probably as small as you wanna go. I use a Straight Line Jumbo Buster tube that's flat and has handles in the middle of it which come in very handy. Looks like they've changed the design of it, though.
If this is your first time out, you might consider putting your rider a little further away than normal. Then as you guys get more comfortable you can start to experiment with different rope lengths.
As far as the housing is concerned, you should double and triple check that the o-ring is seated correctly (I like my Aquatech because I can see the rear o-ring to make sure it's all good). Then hold it under water for a bit and see if any leaks are getting in. Then check the buttons on the housing to make sure they're all working. Sounds a bit tedious, but it's a good habit to get into.