Is Marketo compatible with third part AB test tools like Optimizely? We would like scale up testing LP elements?
I guess the question would be: What does "compatible" mean exactly? But you can certainly put Optimizely code onto Marketo landing pages:
This also depends on what you mean for "ramp up". You can find an overview of the basic options for A/B testing with Marketo landing pages on the Marketo Docs, or some extra suggestions in this blog post: https://chapmanbright.com/resources/blog-post/a-b-test-everything/
Regarding Optimizely, it is a great A/B testing tool but I have only used it before on the main website, not on Marketo landing pages. No integration required in that situation.