Hi Gita. Welcome! Here are some versions of answers.
Can I send my manuscript to a few publishers at the same time?
Yes, most publishers (and agents) accept non-exclusive submissions, meaning you can sub to several at a time. Just make sure your manuscript is really, really ready -- you only get one shot per publisher/agent per manuscript.
Is it necessary for my picture book to contain 500 words or is it ok to have less than 500?
Many editors and agents PREFER short picture books -- under 500 words -- as long as everything that needs to be there for the story is there.
How do you prevent someone from stealing your ideas?
Publishers and agents don't do this -- if they do it's so rare I've not heard of it. Usually ideas are not easily stolen because writing books themselves is HARD and everything is execution. But if you have a very high-concept, commercial concept, you might not want to publish the pitch widely on the Internet. I don't know writers who steal ideas, but there are some unscrupulous people out there somewhere.
Is it necessary to be an excellent writer in order to write a book? I ask because I feel that I am a good writer, not an excellent one, but I have several great ideas.
Answers will vary, but I don't think you need to be naturally extremely gifted in order to produce quality, publishable work. You need some basic talent and feel for language -- the rest is craft that can be learned through study and work.
Is it ok to write two books at one time, especially if they don?t have any correlation to each other?
Every writer has a different method -- some are manuscript monogamists, and others can write several books at time. I'd say that many of us have manuscripts in different stages -- planning, draft, idea. If you write PB, it's best to develop at least three solid manuscripts if you want to approach agents. It's best even if you want to approach publishers, since the skills you learn on each can be applied to drafts of the others.