Can you just walk around with general admission ? Or do you have to follow a tour guide?

You have to follow a tour guide and afterwards you have a general admission to half of the place.

Yes

Actually, there are general admission tickets that let you look without a guided tour. From their website: Can we walk around the Neon Boneyard by ourselves to see the signs? Yes.  We do offer general admission tickets to the Neon Museum Boneyard on a limited basis so you can visit the signs at your leisure.  Museum interpreters are stationed throughout the yard to answer any questions you may have about the establishment of Las Vegas, the growth of downtown Las Vegas, Fremont Street and Las Vegas Boulevard, the role of small businesses in the community and the use of themed properties. Visit the tickets page for availability.

These are guided tours. They allow about 30 minutes at the end of the tour so you can walk around and look at the stuff in the main courtyard, but you are not allowed to go into the rest of the boneyard unattended