Similar to my last question, I am hesitant to send settlers or place a recruitment beacon at Graygarden due to fears of negatively impacting future quests and/or dialogue and/or relations with NPCs/factions. Are there any reasons that I should leave Graygarden's inhabitants to the original six Mr. Handy robots present upon acquisition or can I start moving some people over?
No, there is no impact on the main story but obviously having additional people and further developments on site will aid in any "protect this settlement" mission as they come up.
Graygarden settlement has a lot of area to build in as there is a section of elevated highway within the settlement that you can build both on and under.