Any way you could post a pic of them? Might be easier to tell for sure what they are if you have someone else's opinion.
When they get full grown you will need to get a bigger tank. There is no question about that but the good thing is that you will have plenty of time before that happens. If you plan well and save your money specifically for a tank change-over you should have no trouble keeping them if it is indeed what you desire. And, if all else fails, turtles are often kept in 50 gal. Rubbermaid containers (with heater, filter, etc.) until a tank or a pond can be provided for them. There really is nothing wrong with a setup like this except for how it might look in your house ... but it is much cheaper in a pinch than buying a tank.
As for the UVB light they are able to go a little while without it as long as you can supplement as much as possible with natural sunlight ... if you can find some time when it is warm enough outside they can get sunlight that way until you can afford the UVB fixture. If you are saving money for the light+fixture you could save a little by getting a UVB coil which can screw right into a regular light socket. This should cost you about $25-30 depending on your source.
You are right about them needing a more varied diet. Reptomin is about the best aquatic commercial food that you can get for turtles but it isn't enough nutritionally. Also, it has a high protein content which can be bad for the turtles if it is fed in excess. Feeder guppies and rosy reds are great -- I recommend you get a small 10 gal. (or a Rubbermaid, again
) and breed them yourself. It isn't hard at all and they breed incredibly fast so it is much cheaper than buying them from the petshop all the time. Sometimes fish are really inconvenient so there are a number of other good choices as far as meats go as well: crickets, mealworms, etc,
brine shrimp, etc. In a pinch you can also offer cooked chicken and tuna. Sliders also appreciate greens. Dark leafy lettuces can be offered. Aquatic plants are the best choice -- anachris, duckweed, etc. are good. Variety is the key.
Sexual maturity is more dependent on the size of the turtle rather than age ... you will have to provide a nesting site for a female irregardless of whether or not she mates as she will produce eggs no matter what. It isn't hard in a tank, although a natural setting would be preferred, of course. The easiest method involves you putting a Rubbermaid in the tank with a substrate mixture for her to lay them in. You just simply lower the water level to the brim of the Rubbermaid for the duration of her need. You should be able to tell when she is ready for this because her behavior will change -- she will become frantic, she'll try to escape the enclosure, she'll suffer a loss of appetite ... There's more to it than that but no point in going into it right now.
It's completely normal for them to try to bite you ... aquatics aren't known to be very nice.
Listen, the bottom line is to keep asking questions when you need to. Don't feel bad -- keeping turtles is complicated and especially difficult in the beginning. You are doing great already by asking the right questions and being concerned about their well being! It's tough and there is always something new to learn! But it is extremely rewarding and I encourage you to stick with it. I think you are going to do fine.
Females will be naturally bigger full-grown. Males will have very long nails on their front legs and a larger, longer tail. 
