Say I have a I IV V progression in the key of Em, so that would be Em A B, correct me if Im wrong. Does it matter what type of chord I make the chords in the progression. Like could I make it Em Am Bm, or E7 A7 B7 etc. ? Thanks in advance.
Yes, it matters. In E minor (as your example) an Em7 chord will fit, a standard E7 will not.
You're wrong. (sorry if that came off harshly)

Em, A, B is i, IV, V. It's nitpicking, I know, but when using roman numerals, lower case means minor and capital is major.

E major scale = E, F#, G#, A, B, C#, D#, E
E minor scale = E, F#, G, A, B, C, D, E

I - E = E, G#, B
IV - A = A, C#, E
V -B = B, D#, F#

i - Em = E, G, B
iv - Am = A, C, E
v - Bm = B, D, F#

So for it to work within the scale, I believe for seventh chords in that progression, it would be I7, IV7, Vm7. It's a minor seventh interval, not a major seventh for the fifth. Because the third is still major, the chord is major. The third gives the triad is major or minor characteristic.

I7 - E7 = E, G#, B, D#
IV7 - A7 = A, C#, E, G#
Vm7 - B7 = B, D#, F#, A

You have to work within the key mostly. It takes a bit of work at first, but it comes with time. I think what I've given you is right, but I'm a bit out of practice, sorry.
No, but you could have Em7 Am7 and Bm7, as they all conform to the key of E minor. ^Thanks Maet it wouldve taken me ages to remember the key notes without your post
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