If you want to use your bike to get around with, then a geared bike with bigger wheels is better. It will be faster, less fatiguing and offer greater rider comfort. But if you want to do BMX style riding, with jumps and tricks and that, then a BMX is a better choice.
A BMX style mountain bike typically has a smaller frame, sturdy construction, knobby tires, and a single gear system. These features make it lightweight, durable, and easy to maneuver on rough terrain. The benefits include better control, agility, and the ability to perform tricks and stunts.
Yes, if you can find the correct bottom bracket for the frame.
There are MTBs with 20" and 24" wheels for smaller riders, which probably is the closest thing to what you're describing. These wouldn't be BMX bicycles though, since BMX bikes don't have gears and have a taller handlebar than a mountain bike.
It'd be a fairly strange BMX if it has a gear shaft. Most "real" BMXes are single speed, their driveline consists of cranks, a bottom bracket and the rear hub. No gear shaft there.
It will if you have bmx (or bmx style) cranks.
I have no idea what you mean by "gear bike", so the question can't be answered.
No such thing. Trials is a sub-sport of mountain biking, often called "mountain bike trials". It's a sport similar to motorcycle trials where one takes a light bike and hops around an obstacle course. You might have heard "BMX" and "trials" in the same sentence because a BMX bicycle can essentially make a good trials bike with minimal modifications.
Two, but this is not the same as how many speeds the BMX bike in question has. A BMX bike has a sprocket in the front, and a cog in the back and each is a gear, but is considered a single speed bicycle because two gears and a chain are required for transmission of rotational force from the crankset to the rear wheel.
When choosing a mountain bike gear shifter, consider the number of speeds it offers, the ease of shifting, the durability of the shifter, and compatibility with your bike's drivetrain.
It's more a question of "different" than outright "better".If you want to use your bike to get around with, then a mountain bike with gears and bigger wheels is better. It will be faster, less fatiguing and offer greater rider comfort.But if you want to do BMX style riding, with jumps and tricks and that, then a BMX is a better choice.
you can find a BMX bike in halfords
As a general rule if two bikes are in the same price range, the smaller of the two will be lighter. This because there is less material used in the smaller BMX due to it's much simpler construction. But, when you head for the high end, a flatland or racing bike is going to be much lighter compared to a mountain bike.