Onlar ___ mı/mu?

What is the difference between the mı and mu if they are both in reference to the pronoun they?

mi/mı/mu are the suffixes that make the sentence they are added question..

Tarık komşularını çok sever. / Tarık loves his neigbours very much...

Tarık komşularını çok sever mi? / Does Tarık love his neighbours very much?

- mi/mı/mu are always added to the end of the sentence

--i m not sure that they refer to the pronoun "they"..

--- the difference between mı/mu stems from the sound harmony...

I hope this question is related, I dont meant to "threadjack"

Essentially I am asking if you use the same suffix -mi to ask both "Does he?" or "Is he?" or 

Refering to personal interrogative question tags (-miyim?, -misin?, -mi?, miyiz?, etc)

lets say you wanted to write for example; "Does he swim everyday?" Would that be "O her gün yüzer mi? " ?

Now would you use the same suffix as if you were saying "Is he swiming everyday?" Though after consideration, both sentences can mean basically the same thing in English. 

mi/mı/mu are the suffixes...

It is not a suffix but a word, that´s why it is not added to the word stem.

...that make the sentence they are added question..

It has some other functions as well. Here are two of them:

1)Used to harden the meaning:

Şirin mi şirin bir kasaba 

2)Used as a conjunction

Ezan okundu mu doğru camiye gider. 

Tarık komşularını çok sever. / Tarık loves his neigbours very much...

Tarık komşularını çok sever mi? / Does Tarık love his neighbours very much?

- mi/mı/mu are always added to the end of the sentence

It is not always placed in the end of the sentence:

Ben mi söylemişim?

Ahmet mi geldi?

--i m not sure that they refer to the pronoun "they"..

--- the difference between mı/mu stems from the sound harmony...

mi/mı/mu are the suffixes...

It is not a suffix but a word, that´s why it is not added to the word stem.

...that make the sentence they are added question..

It has some other functions as well. Here are two of them:

1)Used to harden the meaning:

Şirin mi şirin bir kasaba 

2)Used as a conjunction

Ezan okundu mu doğru camiye gider. 

Tarık komşularını çok sever. / Tarık loves his neigbours very much...

Tarık komşularını çok sever mi? / Does Tarık love his neighbours very much?

- mi/mı/mu are always added to the end of the sentence

It is not always placed in the end of the sentence:

Ben mi söylemişim?

Ahmet mi geldi?

--i m not sure that they refer to the pronoun "they"..

--- the difference between mı/mu stems from the sound harmony...

So if those words like "ile", "ise", "idi", "imiş" are treated as suffixes why not "mi/mı/mu/mi"?

Because that mutation hasn´t happened yet, and "mi" still is a word. When in the future you are able to append it to the words, it will be treated as a suffix, not now. Btw the point you miss is that "ile", "ise", "idi" and "imiş" are categorical different words than mi as they all contain auxiliary verb "i". Maybe,who knows, "mi" will never have the same fate as them.