Well Joel with extra lites there maybe be just a touch of losseness in the nut area, but probably not enough for most folks to notice, the mediums will give you more volume then you would have with extra lites other then that they shouldn't have any real adverse effects of your Martin.
Here is a nice little article written by someone regarding strings and so forth, I don't hold with everything he says but wll one should get as many opinions as possible.Ship http://www.flatpick.com/Pages/News_And_Reviews/Gear.html#ST1
This kind of question has been popping up a lot lately. Too light can affect the intonation. Martin's recommendations are based on the tension of the strings exerted on the guitar. Excessive or too little tension one way or another will have an effect on how the guitar responds and the intonation along the fretboard.
My triple aught 16 came with 12's... Just for experimentation I put a set of 10's on it and it was fine... Intonation stayed perfect and it felt good 'but'... I noticed the tone was thinner... so a day later I switched back to 12's... I also use Elixir's on it instead of the Martin's they recommend...
(sure did like the feel of the 10's though)
I was reading about Newtone acoustic guitar strings. They have what are called round core strings. Based on the description on their website the strings have lower tension and easier playability. This allows for strings that are the same gauge as a med or light set. I haven't tried these yet but will likely pick up a set next time I'm out for a string run at the store.
Putting the extra lights on it won't hurt the guitar of course, but if you don't get it set up for that you may have some rattling when you strum even moderately heavy with a flatpick...so a new setup may be in order after you string it up and see if you like they way it feels...also the lighter strings will rob you of a little volume. I use lights on my 1976 Martin D-28 to keep the neck stable, especially since there is no truss rod! I find that the amount of volume I lose is really negligible. Let us know what you do and how it comes out...we just love this kind of "stuff" here!
About your Epiphone...yours has a completely hollow body right, unlike Gibson thin bodies with the solid block down the middle. I bet you get a great open sound from it...it was John Lennon's choice of course! I have an old Gibson Les Paul Signature which I love the sound of..it is not a solid body like a typical LP, but is like a weird ES-335. Here is a picture:
my Casino is completely hollow. Years ago, I had an ES-330 and I loved it. When I started looking for an electric again, I played the Epi Dot semi-hollow and the Casino, and it was no contest. Mine is blonde like Lennon's ended up.
I like that hollow LP, it looks like they borrowed that body for the Epi's Jack Casady signature bass.