I have the same problem.
Bought a maitreya body, and not even all clothes fit right. Some let parts of the body showing. Quite a lot more that I was expecting to happen.
So I eventually try a lot of demos, to only buy a couple of things, and sometimes it makes me sad as some that I have to trash are really beautifull.
As for underwear under the clothes, only a few could fit right. Unless those that go like a tattoo.
I'm new in SL, so not sure if there is anything else to do.
Btw, when we change outfit, does the tattoo layers change too? And how do I remove a single tattoo?