First of all I'm going to say that I'm a big believer that your tank is your tank. I believe that if your water parameters are good and your fish act healthy, then do what you want.
I keep and breed all types of platys along with a variety of other fw fish and shrimp. I keep all my tanks at 78. Platys are fairly active fish, so I would say that if they are not swimming most of the time then they are too cold. Plus without a heater, how are you keeping your water temp constant? I have a even smaller tank then your 7 or so gallon tank and I bought a 3-10 gallon heater on ebay for less than ten bucks. I would go for a heater... Again this is your tank so ultimately don't let people bully you into doing something you don't want to do.
As far as tank mates: IMO 7 gallon tank is too small for both platys and danios. If you want more fish I would say maybe you could add another platy or two. The reason I say this is because platys are pooping machines. I would get some shrimp. I have read that shrimp do ok in cooler water, just make sure it's consistent. Shrimp have really small bio loads (poop) so you could do quite a few if you stuck to the smaller types like red cherry shrimp or any of their cousin breeds. I have red and yellow and they are a beautiful pop of color. There are also bright blue, green, brown, and mixes. My shrimp were shy at first, but now that they know the platys don't consider them lunch, they are all over the place. Red Crystal Shrimp are really pretty, but I don't have any so I don't have any thing to add about personally. Another way to go is Ghost Shrimp. IMO ghosts aren't as pretty because they are clear and kinda look like underwater bugs, but they are great helpers with keeping the tank clean. Most people say they don't, but I have watched my ghost eat platy poop. So if this was my tank I would add 15 Red Cherry or Yellow Cherry or whatever color you're willing to pay for and three ghosts. Ghost will eat RCS shrimpletts, so if you want to breed your pretty shrimp I wouldn't get ghost. Plus if you do want to breed shrimp, don't get more then one color or they will cross breed, which will breed plain brown shrimp.
If you have trouble with algae, you will have to warm up your water to have helper fish. If you do warm up your water you could then get 3-6 otos. They are adorable mini sucker fish and do great with platys and shrimp. Plus they swim all over the place and add a lot of fun. But don't get them if you don't have algae. That's all they eat, so they would starve unless you feed them algae tablets which some won't eat because they are wild caught.
And hey, take it from someone who has listened to LFS employees and been misinformed, some LFS employees don't know swat about fish. Some do; some love fish as much as we do. But don't take their advice and run with it. The people on this forum may sometimes seem bossy, but some have been keeping fish for years and some are like me that just do tons of research. Hope this helps!
