Publishing as a IEEE letter vs article I am writing a paper and am planning to submit it to one of the IEEE journals. I wrote the full paper and then realized that it is possible that the particular journal requires experimental validation of any modeling results. It doesn't explicitly state this in the website, but this is what it seems like from reading multiple papers.

I also came to know that it's possible to publish the article as a letter. I will have to reformat the paper and cut down the size by 1/3rd. It seems that the rigor of the data required to publish letters is a bit lax compared to full scale articles: Letter vs article, What is the difference between an article, perspective, letter, and review?


Do you think the requirements for publication of a letter is more lax compared to a full article?
If I were to submit it as a full paper, and it gets rejected because it doesn't have sufficient amount of experimental validation, will I still be able to submit as a letter?
In your question, you mentioned that you "wrote the full paper and then realized that it is possible that the particular journal requires experimental validation of any modeling results." 

In other words, you seem to be missing a key piece of content in your manuscript that a full-length paper would be expected to have. And now you want to explore the possibility of trimming your paper down to submit it as a Letter, in the hopes that a shorter paper might help you to avoid having to do any experimental validation of your models. I could be wrong, but it seems that you might have the wrong idea about what a Letters paper is all about (at least as far as the IEEE is concerned).

Thus, before answering your questions, I think it will be a good idea to look at what the IEEE says regarding the Letter format, and what their intended usage is. [Also, since you commented on the possibility of submitting to the IEEE Journal of Microelectromechanical Systems (JMEMS) in particular, I will incorporate that venue in my answer below.]

From the IEEE JMEMS site, we find that:


  The Letters format is meant to encourage rapid publication of original and significant contributions that are limited in length (three or fewer journal pages), but appropriate for early dissemination.


Further, from the same IEEE JMEMS site above, we find that those seeking to submit their Letter are required to do the following:


  All submissions to JMEMS Letters are required to include an accompanying submission letter that puts forth persuasive arguments supporting the need for rapid publication. Possible supporting statements for these arguments might be built using: 1) references to recent literature that illustrate how the Letter addresses an already existing and pressing problem, and/or 2) "impact" statements outlining timely ramifications of the work, and/or 3) clear arguments supporting the case that the Letter points the way toward significant and meaningful changes in previous technologies, materials, or designs. 


Note that nowhere in the information above do we find that the Letter format is suitable for those authors who are simply looking to trim down their full-length paper worth of research findings because they do not have key pieces of expected content for a full-length submission.

So, let's get to your questions: 


  Do you think the requirements for publication of a letter is more lax compared to a full article?


No, the Letters option should not be viewed as an easier path to publication. (Surely, if it were easier to publish in the Letters format, then everyone would do it.) 

In fact, as noted above at the IEEE JMEMS site, authors submitting Letter manuscripts are expected to justify why their work should be rapidly disseminated, and this required justification is on top of having a manuscript which makes a coherent case for the research findings presented, is a good fit for the journal, etc.


  If I were to submit it as a full paper, and it gets rejected because it doesnt have sufficient amount of experimental validation, will I still be able to submit as a letter?


If you were to submit your manuscript as a full paper, and it was rejected, say, because it doesn't have a sufficient amount of experimental validation, I doubt that submitting it for consideration as a Letter would be successful.

Why? 

For IEEE publication venues in which full-length papers and Letters are both handled by a single venue (e.g., the IEEE JMEMS), the full-length paper and Letter submissions are overseen by one Editor-in-Chief (EiC).

Thus, if you submitted your full-length paper and it was rejected on the grounds that your paper was missing key pieces of expected content, we have to assume that the EiC (for IEEE JMEMS, in your particular case) would already be aware of your manuscript's submission-and-rejection history.

Surely, any EiC at any journal of reasonable quality would view this course of action on your part as an attempt to "game the system," and they would thus frown on this strategy rather heavily.

Going forward, I suggest the following course of action:


If I've totally misjudged your situation, and your paper really is a good fit for the Letters format (as outlined above), then go for that.
Else if your manuscript is better-suited as a full paper, then either work on the experimental validation of your modeling results, or find another venue where such validation is not seen as a requirement for publication.