If I was to buy a headjoint from another brand, say Powell, and use it on my intermediate flute, would it fit? and if it didn`t, would a repairman be able to adjust it to fit my flute?

Hi, I can`t see why not.

Some of my flute friends always use different headjoints with the same flute body in different situations eg playing in concerts or solos.

They should fit in each other.

Great precision is required for heads to fit sockets well.

It is not common for a different brand of head to fit without a fitting process.

Any worthy technician should have the specialised tools to git the head to the flute unless on is an unusually small or large diameter. The fitting should take the technician say between 1/2 and 5 minutes.

Thanks, out the 2 Headjoints listed below, which would you guys perfer and why?

Burkart Acton 998 Silver Headjoint with a Platinum riser

or

Powell Soloist Style 14K Aurumite Headjoint with a Platinum Riser

I would say just go with the headjoint that YOU like the best. Nobody can make the decision for you. Everyone wants a different sound and everyone has a different embouchure and facial structures so no brand is "the best". I would say try the two and pick out the pros and cons of each one and decide which one you feel suits you the best.

Sound advice.