Is mucho grande okay?
Mucho is used for nouns while muy is used for adjectives. Mucho in English would be "a lot".
Wouldn't the word "grande" also serve as the word "Great"? For example "Para grande personas" (for great people)???
Yes it does also mean 'great'. Your example, however, would be "Para grandes personas".
'Muy gigante', does not work. Is there a reason?
Just like in English, big and gigantic are different words. They mean a similar thing, but they're different. It's the same in Spanish
Yes, muy gigante tambien se podría usar pero no es muy común usarlo