To help your plant be able to support itself longer (although still not if it gets too long) you can give it some potassium sulfate. It really strengthens the stems (without killing microbes in your soil). Calcium and silica help to strengthen plants, too, and calcium goes hand-in-hand with potassium. I've used basalt rockdust for calcium and silica, but there's probably a more ideal product for tomatoes there (rockdust is more of a long-term thing). I know they initially like greensand a lot more than basalt rockdust (which might work for the silica, but you may need a calcium source, like maybe a little gypsum).
To prevent your plants from growing too fast, you can plant them in clay or clay loam soil (make sure you don't bring insects or diseases in your house with the soil, though). Plants grow very slowly and steadily in our clay-loam soil, indoors (at least at my house), but they still fruit in it. I imagine they would grow even more slowly in pure clay. If you sterilize some outdoor soil, I recommend adding some worm castings or something to put some good microbes back in the soil.
Pruning your tomatoes can help. I'm guessing you have indeterminate tomatoes.
Small pots and multiple plants per pot can result in smaller plants, but it may have a negative impact on fruiting. Plants may also need to be watered and fertilized more often.
If you're not partial to the specific plants you're already growing, variety counts. Things to look for in a variety for indoor use include these:
- Size. Some varieties (like Micro-Tom, Tumbling Tom, etc.) are a lot smaller and still produce a reasonable amount of fruit. That doesn't mean they'll do it indoors, though, even if they fruit well in outdoor containers, but you never know until you try it. There are several kinds of tomatoes. Indeterminate: they grow and fruit indefinitely; they vine out and can get big; they will probably require significant pruning if kept indoors. Determinate: These are typically smaller/bushy, fruit once, all at once, and die. Semi-determinate: These often fruit and grow indefinitely, but slower to much slower than indeterminate tomatoes. Dwarf-indeterminate: I believe these are just smaller indeterminate tomatoes. It should be noted that not all tomatoes marketed as determinate behave determinate in every fashion, and semi-determinate plants sometimes grow pretty big. Determinate tomatoes are said not to do as well indoors; my guess is it's probably because they only fruit once, and conditions might not be right when they're ready. (This also may have something to do with there not being regular seasons indoors.) It sounds like dwarf-indeterminate tomatoes might be ideal for your situation.
- Parthenocarpic (will set fruit without pollination; they also set fruit in a wider variety of temperatures)
- Tolerant of heat and cold (indoor temperatures are often too warm for tomatoes, despite being tolerable for humans; the daytime indoor temperatures are usually fine, but they need to get somewhat colder at night if the daytime temperatures are too warm to be their constant temperature.)
- Drought-tolerant (This is just a good precaution, especially if your container isn't large)
- Look for varieties well-suited for containers. This is often advertised with the variety.
- Shade-tolerant (light is usually more limited indoors; so, plants that need less of it are a really good idea; I would guess that cold-tolerant tomatoes might be able to withstand a little less light, too, but that is just a guess; when I say shade-tolerant, I don't mean tolerant of full-shade; I mean, partial shade)
- Productive (productive plants are probably better at utilizing limited resources)
- Tolerates humidity (if you have a lot of plants, odds are, your growing environment is humid; if you just have a few, it's probably not a big deal)
Without regard to the amount of soil, light, water and fertilizer needed, here are some parthenocarpic plants that I would recommend investigating most, out of the parthenocarpic ones:
Golden nugget will probably be the easiest parthenocarpic tomato for your situation, and since it's fairly small and as it has small fruit, it should have fairly modest requirements. It's supposed to be rather productive.
Aside from parthenocarpic kinds, a lot of peppers can fruit indoors just fine. Finding the right ones is the thing, though. Hot peppers probably set fruit inside more easily, usually.
Some other tomatoes that I would suggest looking into include these:
- Glacier (semi-determinate; heat/cold tolerant)
- Coldset (determinate; heat/cold tolerant)
- Payette (plants described as polite and ornamental; should have above average heat-tolerance)
I believe Glacier and Payette have been tried in containers before with success. I'm not sure about Coldset. I'm not sure if these have below-average light requirements, however (they may need a whole lot of sun for all I know). Anyway, these are some experimental ideas. There's not a lot of knowledge on the Internet as to which varieties do best indoors (although container knowledge is another matter). So, experimentation should help. I've gotten indoor fruit (not loads of it, though), on Galapagos Island (Solanum cheesmaniae); I only had it in a 20 fl. oz. foam cup, though (clay-loam soil). I imagine you could get more out of it with more soil. It has thin branches, but it's indeterminate.
I've had Aunt Molly's ground cherries do well indoors (even crowded with multiple plants in an 18 fl. oz. container; I imagine they could do much better in a bigger one, with one plant), if you like ground cherries (the plants are quite small). The fruit is supposed to be ripe when it falls off the plant.
You can find some more traditional tomato varieties for containers in a web search. I have no idea how they do indoors, other than their suitability for containers.
Smaller tomatoes are supposed to do better indoors, more often. I think it still depends on the variety, though, from my experience.