Pleco's are tropical fish, so if your pond has a sub-tropical temperature, the pleco will not survive in the pond for very long.
Only if the Pleco is big enough
when water temp is same as where ther at now,
Yes, until they outgrow them. Then put them into an outdoor pond.
You CAN put one in a pond, but it will slowly sink.
Make sure the pleco is larger or of the same size as the terrapin, else it will get bullied by the terrapins, pleco's tail is a irresistible bait for the terrapin.
My guess it that it is not going to be a happy little turtle! I would recommend a MUCH larger tank or a fenced-in outdoor pond!
Provided the water temp does not get below 75F or above 85F. Oscars are hard to see in a pond. They evolved to blend into the background and disappear when viewed from above.
Some of the Pleco family grow quite large and should be kept with other largeish fish like Jack Dempeys and Oscars. Others are considerable smaller species but they will be OK with most of the Tetras, Barbs, Danios, Dwarf Cichlids and Anabantids.
Yes you can put your bose outdoor speakers in your garden. You can put these anywhere in your yard.
On ground near a pond
Better than what? I don't know exactly what you mean or why you ask, but bettas (assuming that's what you mean by fighting fish) need clean, chlorine-free water that is above 76° and filtered and at least 5 gallons to swim around in. You can't keep them in an outdoor pond and water taken from a pond is likely to have diseases that will kill them.
If it's large enough. If the Pleco is too small, the Jaguar Cichlid will try to eat it and the Pleco's plating and spiked fins will either choke it going in, or injure it comin' out! Either way, they will both die.