The one in the first pic is a Yellow Bellied Slider (YBS), the other one is probably a Cumberland Slider, or maybe a YBS x Cumberland cross.
YBS typically have that big yellow "S" shape behind the eye which the first one has, so there's no question about that.
Cumberlands don't have the S and have a series of yellow stripes instead, the second one almost has the S shape but it's not complete, and it has some broad yellow stripes and some finer ones, which makes me think it may be a cross.
YBS and Cumberlands are closely related subspecies of Trachemys and frequently interbreed, especially in turtle farms where most of them come from.
Hi, We have two Red bellys, they are aroung 1 yr old and we keep them in a 3 ft tank. We have including a basking area with a temp of around 31, and the water is kept at 25. We have an external ehiem filter with built in heater, this filter is big enough to keep the water very clean and avoids having to do water changes every week. We have a fine sand as a substrate and although people say that it harbers bacteria, the terrepins move around so much, they disturbe the sand which releases the muck into the tank which is then digested by the filter. They also enjoy digging in it. We also have a UVB repti glo 10, as one of our little fellows started to suffer from soft shell which is now on the mend. Be carefull with thier diet and ensure they get enough calcium by feeding kale etc. Try to keep away from the all in one food sticks as this is what helped cause the soft shell, so we only feed this after soaking the sticks in calcium. We aslo have a few tetra neons in the tank which help eat all the small particles of food and muck. The terrepins seem to leave them alone as the tetras are fast, but i am sure they will eat one of them soner or later.
We have aslo tried red crabs in the tank, they added a good source as they scavenge a lot, and will happily eat anything that lays on the bottom of the tank. Hopefully that will help you, as we learnt the hard way by listening t a crap pet shop who told us they didnt need lights etc and also said they did not need any type of heat etc!!! Thsi is ne of the reasons we suffered from soft shell.
The problem with sand substrate, other than harbouring bacteria, is that the turts can easily ingest it accidentally which can cause compaction, which is often fatal.
Also the very fine sand particles can get sucked into the filter and cause premature bearing wear. At least with Eheims the bearings are usually ceramic so it's less of a problem than with cheaper filters, something to watch for though if your filter starts to sound noisy.
It's fine to keep them together, their requirements are virtually identical and they should get along well together. Turts are not like torts in this respect, it's usually OK to keep different species together provided their habitat and feeding requirements are the same.
Bothe YBS and Cumberlands can reach 12" or so for females.
You can't tell what sex they are until they are a few years old, usually by the age or 3 or 4 it's pretty obvious but earlier than that would just be guessing with most species.
Adult females of all species are bigger than the males when fully grown, like torts the females have shorter, thinner tails and the cloaca is closer to the base of the tail, males have longer, thicker tails and the cloaca is nearer the tip. This difference in the tails is more pronounced in some species than others, in Maps for instance it's very obvious and from quite an early age.
Males of most species also develop longer front claws, male sliders in particular have much longer claws than the females, up to an inch long for a fully grown male and there's no mistaking them, in other species it may be less obvious.
Cheap tanks are available from many sources, you'll often find that the bigger the tank the harder it is to sell and therefore cheaper, but you have to be able to collect it of course! I've seen loads of perfectly good 8' tanks sold really cheap or even given away, but you need a van or trailer and a team of strong guys at each end of the journey.
Try Freecycle, I've had a few good tanks from there, for free of course. Some nice big tanks come up on eBay, and something like a 4 x 2 x 2, or 6 x 2 x 2 usually fetches well under £100, I saw a 4 footer go for just £16 last week. And the fish section of Pets Classifieds is another good place to look.