So I know that settlements are capped at your CHR +10 with regards to "natural" growth, but can you exceed that by sending settlers from another settlement? Say I'm capped at 18, can I send another 10 from other settlements and it'll stay at 28?

No, you can't. You won't be able to select settlements on the list if they are at the limit.

If you put on gear or use chems That has chr boost then you can send that boosted amount then remove the clothing and the settlers will stay I had 23 at one time using a suit and hat to boost my chr by 3 points. I could have went higher if I used the chems that also add chr. Say you have 10 chr + 10 plus 3 for Reginals suit plus 1 for the postmans hat you can have 24 plus the chr for drugs but im not sure the boost on them. I think one gives you another 3 but not sure

Yep, take chems and wear clothes that increases your charisma. That's the only way to increase the population above the "natural cap".

- Graqe Mentats increase charisma by 5.

- Suits / some dresses increase charisma by 2.

- Some hats / some dresses increase charisma by 1.

- Black rimmed glasses increase charisma by 1.

- There is a unique dress / suit that increases charisma by 3.

Note:

- There is a wandering weapons trader; Smiling Larry; that you can recruit as a level 4 trader, but your settlement needs to have at least a population of 30.

- All SPECIAL attributes can only be increased naturally to 10. However, they can all be boosted permanently to 11. In order to do this you must raise the attributes to 10 by leveling up. At that point when you pick up a bobblehead that increases an attribute, it will be raised to 11. If you have already picked up an attribute boosting bobblehead before getting it to 10, then you cannot go past 10.

The Wiki suggests you can get +16CHA from items, chems and alcohol. It lists graqe mentats +5, certain unique clothing +3 (common clean clothing +2 instead), hats +1, eyewear +1. Beer gives +1, Day Tripper +3 and X-Cell +2.

Better get some Addictol too...

I have yet to figure out the system. I had one settlement that got up to 33 settlers, two had over 25 and the rest stuck around 16.

33 settlers is alot considering how the number is devised.

I made it to 17. Agatha's dress, fashionable glasses, formal hat, bobble head and a bottle of beer. Might be that some chems could raise it even further. I'm not sure about stacking alcohol and chems.

My understanding on Smiling Larry was that you had to have at least 30 settlers total.

Two settlements of 15 should meet that.