So, on my latest playthrough I noticed that even my biggest settlement is struggling to get more that 21-22 settlers (oddly, the most populated settlement I have is Jamaica Plain and that only has 21) so I was wondering if that is the limit to which a settlement will grow?

I was also wondering how many different "types" of NPC / Creature etc there are?

For example, I have met "normal", Stalker, Hunter, Predator, Glowing and Deathskull Radscorpions and "normal", Stalker, Reaver, Wthered, Charred, Glowing, Putrid Glowing and Bloated Glowing Ghouls (plus legendary versions of most of them).

Do we know of or has anyone made a list of all the types of NPC / Creatures there are in the Commonwealth?

high charisma helps for all i know

try this:
http://fallout.wikia.com/wiki/Portal:Fallout_4

Awesome, didnt know about that, that answers those questions then! Now I know not to bother building more than 21 beds in my settlements in future!

You can get more settlers at a settlement if you want to.

For example i sent a few to Egret Marina just for the fun of it.

picture here.

http://ocau.com/pix/et4gm

When a settlement is full you can go to another settlement and send some more settlers over.

To do that just before entering workshop mode and moving the settlers temporarily boost your charisma by putting on a suit, hat, glasses or other clothing that boosts charisma.

To send even more, whilst wearing the clothes, take some drugs like graqe mentats and also drink a charisma boosting brew.

With your other question about all the characters apart from the wiki there is a vault dwellers survival guide that has been published. The guide lists all the npc variants and some information about them in a fairly easily readable form.