On the topic of LLC vs. corporation generally, please see the post at the link below.
While consolidating different businesses under one entity may save some overhead (such as annual filing fees), there is a significant risk that a liability incurred by one business may result in loss of the other business. Accordingly, it is common to have two different entities for the two businesses.
The ease of adding more members (LLC) or shareholders (corporation) later depends, to some extent, on how thoroughly you have thought about the process up-front.
The foregoing are general comments; the right choices for you depend on your specific needs. You should retain a business lawyer who is knowledgeable about startups to guide you through the process.
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.
What you are proposing is legal, but may not be practical. For example, i assume you have errors and ommisions insurance for the IT consulting business. You E&O insurance carrier may balk at the idea of having a completely unrelated business, such as a vacation rental business, operate under the same corporation. That's just one example that may become a problem. I recommend you consult with a gray-haired business attorney before taking any further steps.
Attention: This response is based upon general legal theories and may or may not specifically address issues that affect your individual legal matter or situation. It should not be relied upon outside of the jurisdiction(s) in which the attorney is licensed to practice as each state has different laws. Each situation is fact specific and requires comprehensive legal evaluation following a thorough consultation and review of all the facts and evidence available. This response does not create an attorney-client relationship between the asking and answering parties.