Yes, if you can find the correct bottom bracket for the frame.
A bike to use in a triathlon would be either a mountain bike or a ten speed street bike depending on the riding, do not use a bmx bike or you will be sorry.
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.
Probably not. BMX bikes usually have freewheels while MTBs usually have freehubs, and these don't mix. You might be able to use parts from a single-speed MTB on a BMX though.
Depends on the diameters of the bars. Usually, but not always, they're the same and can use the same grips.
No. BMX wheel size is 20" while most common MTB wheel size is 26". The MTB wheels just won't fit. Drop out width is different too.
freestyle, dirt, street
I don't think a pro would use a £80 Bmx bike lol
They are designed for 2 different riding types. If you have the ability the modify a steel, steel composite, or aluminum frame you can do it. But there is a urban sport type of bike out there called a 29er bike.
it matters what you are going to use it for
BMX bike ramps are usually made out of material such as chrome or aluminum. They have to be durable enough for multiple people to use them at the same time.
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.
Only if you're planning to use them.