Cutting back tomato plants
 

I have an indoor hydroponics system. My tomatoes are getting too long. Is it okay to cut them back in order to keep them from getting even longer? I've heard that you're not supposed to do this, but I thought I might ask the experienced ones for advice.

I'm not really sure what you mean by "getting too long." Are they stretching upward to get light, or are they just getting too tall and wide for your setup? I don't think cutting them back will help you either way. If they are stretching upward to get more light cutting them back wont fix the problem. If they are just getting too big for your setup cutting them back can temporary help, but unless your goal is a bonsai tomato plant that wont fix that problem either.

Bush varieties of tomato plants are hybrids that were bred to grow small, and usually grow between 3 and 4 feet tall. Regular varieties can grow very tall, 10+ feet tall as long as the plant is supported.

I think I would train they to grow in a different direction. Instead of up, try using a trellis that is more horizontal, instead of growing them up, try growing them out.

Yes, they are like a vine - you can do a lot with them.

Indeterminate Tomatoes are vining
 

Since it's indoors you can hang hooks from the rafters, tie a string or wire to the stem at whatever you feel is the right hight (somewhere below your lights I presume) and let the rest just hang down from that. I read somewhere that commercial growers use this technique but the allow two main branches to survive and train them along a string in sort of a teepee formation.

Make sure you avoid damage to the stem by using a plant clip or a "Y" connection of some kind. The idea is that the plant will become quite heavy and the point where it is tied off will otherwise get considerable stress.