Enable Multisite on Existing Site?

Before I ask the question(s), here’s is what I currently have and hope to achieve. I have an existing site being worked on with “theme A”. I am deciding that I would like to create a Landing Page using “theme B”. In order to do that, I have read that it requires either installing another WordPress install in a separate directory or using Multisite.

I have tried using Visual Composer, but I’d prefer paying for another theme to use separately from my main site.

My question(s): Is it too late to install multisite since I have been working on my site off and on for the past month or is it possible to enable Multisite now and then start with the Landing Page? Would it be better to start everything from scratch and enabling Multisite right from the start rather than continuing? Cheers