Let's clear up some concepts:
"Adding a tag" can mean two things:
- Adding an existing tag to a question, or re-tagging
- Adding a non-existing to a question, or tag creation
Looking through the existing tags reveals that there are a few tags for individual persons: like Caesar, Napoleon, Hitler; but not for Dwight Eisenhower or your other suggestions.
What you seem to have in mind is therefore more like case 2: adding newly created tags to the system and some individual questions?
Generally there are a few pointers for general advice on how to use tags here:
What are tags, and how should I use them?
As a general rule, you should avoid creating new tags if possible, and new users are not allowed to create new tags. Even if you have sufficient reputation, you should only create new tags when you feel you can make a strong case that your question covers a new topic that nobody else has asked about before on this site.
Here on History Help Center.
Then there are the network wide meta posts:
How do I correctly tag my questions?
Try not to create new tags. If you create a new tag, that tag is guaranteed not to help your question show up on any subscribed RSS feeds or interesting tag lists. Again, the look-ahead prompt can help with this. Odds are it also means you're missing an existing tag for that topic that would more accurately categorize your question.
Retagging a question is making any edit to a question that changes the tags.
Do not retag a question if you are not going to add value to the question information by doing so.
Do retag questions to use well-known and popular tags that are appropriate for the question.
Why do we tag questions?
Tags connect experts with questions they will be able to answer. If you tag your SQL Server question 'mssql' instead of 'sql-server', odds are it won't get as much attention and may not get a great answer, because frequent users who use SQL Server have favorited the 'sql-server' tag. They know to watch the sql-server tag for questions they can answer, but are not necessarily watching for mssql. A good rule of thumb is that any tag used less than 10 times is probably wrong, and any major language or product tag used less than 100 times is also probably wrong.
Tags are for sorting your question into specific, well-defined categories. Each tag should by itself refer to a specific category. If a tag only makes sense when used in combination with another tag (like '2005' with 'sql-server', 'visual' with 'studio', or '3.5' with '.net'), it's a meta tag.
The above does not mean that you should refrain from creating and adding tags at all. It does mean that you should exercise frugality and minimalism when creating tags.
It is therefore indeed not a good idea to now go on a spree and re-tag-create every question you see might fit. Also for reasons of those questions then all bouncing to the top of the homepage, cluttering the 'really active' posts: questions that are being posed and answered.
As a rule of thumb I'd suggest here that if there are three or better yet five separate questions that would benefit from such a tag you might go ahead with it. I am undecided whether all of your personalities should receive a tag, some may not be that important (yet?) on this site to justify such actions, some might already have such a tag.
Keep in mind that just because a person is connected to the question or a possible answer does not mean the name should appear as a tag then. Tags are to best describe and categorize questions. If that is achieved with a person's name-tag, then do it. Otherwise you might just add another appropriate tag, change one to a better fitting one or leave the existing ones in place. If a post does not already have five tags, chances are high the question might benefit from another tag or two.