I might watch it alongside Makeine, which of them should I start first?

They are quite different, so it’s really up to you.

I find that Makeine is very similar to Makoto Shinkai’s works in terms of art style, soundtracks and humour.

I would say the most similar anime to 100 Girlfriends is Gintama, though season 1 is really just a preview of the bullshit to come.