Same order, Cyprinodontiformes, different species. Mosquitofish is Gambusia affinis and guppy is Poecilia reticulata. Mosquitofish is native to US and guppy further south.
You'll be surprised how cool water in a container will stay given some plant cover like lilies. A few days of record temps won't affect a good size container much. No plants, full sun, black uninsulated container, then sure, a problem.
To split hairs...
Surface tension is no barrier to O2, CO2, molecules. There are 3 main models for air-water gas exchange: stagnant-film, boundary-layer, surface-renewal. Surface tension is unrelated. Tons of papers written on the subject. It is true that stirring water will increase gas exchange because the exchange happens at the interface, but it isn't true that surface tension has to be broken. However, it is also true that surface tension can help create films on the surface, dust for example, which does lower gas exchange.
I know virtually all aquarium writing says that surface tension must be broken for gas exchange, but that has never been true in physics.
Since moving water will increase gas exchange the question still should be whether that increase is needed. Mosquitofish have done well in still water for a very long time so imo additional gas exchange wouldn't be needed. If you want moving water then you certainly should go for it since it is a hobby and you should have fun.
Another option I have used a few times....if you can't bring electric to the water, bring the water to the electric. I've used this for containers and small ponds when I didn't want to see a pump or an outlet. I connect 2 pipes to the container. 3/4" PVC is fine. Run the pipe to the nearest electric. It can be a 100' or more away. Put another container by the electric and connect the 2 pipes to it, add a pump and connect one pipe to the pump's output. The output in the remote container can be brought above the surface if you like, waterfall, fountain, whatever. Water flows back thru the other pipe to equalize the 2 containers.
The container with the pump has to be at the same or lower level as the remote container. If lower the incoming pipe's outlet has to be at the remote container's level, so might be above the pump's container.
You can connect many containers around the yard this way using a single pump. Playing with elevations you can even have fountains in each container.
Initially sounds a bit crazy but it's exactly how many large ponds are plumbed. It's easier and cheaper than running electric and I think it looks better. Plus you get better water conditions, can add filters if needed, etc.
