Many sites in the same VPS

1. Is it safe to host many sites in the same VPS, under the same IP? I mean: if one of my sites has any problem (let’s say, it is in a google’s blacklist), the rest of my sites will have any problem?

Unless you are hosting content that is likely to be a problem this should not be a major concern. This is a very common configuration.

2. Is it recomendable to host the most important sites in an individual VPS to avoid any kind of problem? Or is this a stupid thought?

If you expect problems then this might be a prudent step.

3. Is it possible to host regular sites (like html or php) within Wordpress sites in the same droplet?

You can host these side by side and there may be some ways to host another site in a subdirectory of a WordPress site but I would strongly advise against it. Instead I would recommend creating a subdomain sub.mydomain.com for your second site on the same domain.

4. Last: how can I calculate the size of the droplet I need? I was looking at loadimact, but I don’t understand how to transform all that data into the answer about which droplet specifications I need.

This one is never easy. There is really no way to know exactly what you’ll need without existing data on the sites you’ll be hosting. Many things come into play including the language of any interactive content, the audience both volume of visits and their activities, things like how many database queries may be needed for specific pages can also play a role.

For 30 sites I would recommend you start small and use the resize tool in the control panel if you find that you need more resources.

If these sites are low to moderate traffic I would start off with a droplet with 4GB of RAM at a minimum

If these sites are moderate to high traffic you may be better starting with an 8GB droplet.