I think carambola is a lot more sensitive to cold then figs are...and they need high light all year unlike figs. They are evergreen and do not go fully dormant. I think they grow very well in pots but you'd just have to bring them inside before frost danger and have it in a sunny area of your house...spritzing it with mist every 2 days or so to increase humidity. Those sources are right...young trees get damaged around 27 degrees but I would never risk that if I didn't have to so bring them indoors as soon as there is a frost danger. As for cross-pollination...I know for sure that the variety "Fwang Tung" produces fruit with only one tree and needs no other trees to produce fruit. This is a great nursery you can buy them from.

Here is a link that might be useful: Starfruit nursery

I'm in Ohio and have a 1 1/2 year old starfruit grown from seed. It does very well in pots. In fact, I just repotted it for the third time. Incredible root structure. They do like a lot of light so I don't think putting it in your basement will work. Try a good sunny, south-facing window. Avoid ANY fertilizing the first couple of years. They will overgrow their ability to support themselves. I made this mistake and had to cut nearly two feet from the height. They also will not tolerate even medium strength insecticides. I lost two this way. My last is being treated like gold! They are beautiful plants and it makes me sad to think how nicer it would look today had I not needed to trim it down. Good luck.

I have a nice little starfruit I grew from seeds I got on eBay on the cheap. Like it's been said before, keep it warm, give plenty of light, no fertilizer for the first few years, and spritz for humidity. It gets incredibly dry where I am (We're talking 10-20% humidity during the summer), so I do the latter every evening and I run a humidifier during the winter to make sure it keeps moisture in the air. I have mine under a 65 watt grow light that's on a timer for about 9 to 10 hours a day near a north-facing window because I have no south-facing windows that wouldn't be attacked by kitties, but it seems to do quite well like that anyway.