How to use neocomplete in ftplugin

Question

I want to use the plugin neocomplete only for a few filetypes. For example for Go programming language. So I have the following entries in my ~/.vimrc/ftplugin/go.vim:

...
let g:neocomplete#enable_at_startup = 1
let g:neocomplete#enable_smart_case = 1
let g:neocomplete#sources#syntax#min_keyword_length = 3
...

(My plugins are managed through VimPlug which works flawless.)

Now I have one problem and one question.

Question: Is it bad practice to set global variables in an ftplugin? And why? They seem to work with the exception of the neocomplete#enable_at_startup variable.

Here it comes to the problem: neocomplete doesn't do auto-completion until I enable it manually:

:NeoCompleteEnable

I have to add the following line in my go.vim to enable it automatically at startup:

autocmd VimEnter * NeoCompleteEnable

My question and my problem seem to affect each other. Is there a better way to enable neocomplete plugin only for specific filetypes and to set also different values to the variables for each type?

Greetings

Philipp