I use kettlebells and bodyweight exercises, but I think it's probably a bad idea to try to use them to build frame or internal strength. Doing those (kb's and pushups) correctly (in the traditional manner) isn't consistent with internal strength and the levels of resistance would probably engage local muscle too much.
I use kettlebells and calisthenics to build overall strength for health and fitness, but I consider it completely distinct from my internal strength work. I'm not doing them to build internal strength and don't expect that work to have any impact (except this one...the stronger my legs are the more ultimate power I can generate, internal or external).
I know the arguments that you must not do any weightlifting and etc. if you want to develop internal strength, but I'm skeptical. Nothing from what I know of motor control and learning or skill development and specificity provides a rationale for that...building internal strength is very difficult and requires work every day, an active approach to re-training the body. I don't see how a couple or three days per week of specific strengthening of local muscles makes that effort much more difficult than it already is. That would almost suggest that one's muscles should be weak in order to build internal strength. Ummmm, I don't buy that. It's, imo, the habitual ways we tend to move our bodies that is the impediment, much more than specific, limited things we do to strengthen the limbs and core.
Hey Joseph, I went through this for a while, and I came to the realize that I do have to choose one or the other. One reason is because of time constraints. We're all busy people and we really don't have time to train both methods at the same time. Also, anecdotally, my IS skills finally were improving (I'm not a m4asta though) when I stopped lifting weights. Although it's probably not causative since it can be due to the fact that I started to focus on neijia practice a lot more and doing it more frequently. Who knows?
But anyway, demanding IS practice should improve your overall fitness. OTOH, demanding athletic stuff doesn't improve IS at all (maybe a miniscule bit). So might as well do IS stuff all the time. Besides, all neijia gurus agree about this, so why go against them?