I'd like to give a perspective based on my experience as a user of both the Stack Overflow [r](http://stackoverflow.com/questions/tagged/r) tag and [Cross Validated](http://stats.stackexchange.com), who on Stack Overflow can vote to close as off-topic and migrate. We have a fair number of migrations between these sites. Some crap is migrated, but generally migration works well because the user base of these sites sufficiently overlaps and we've come to a general understanding [[1](http://meta.stackoverflow.com/q/261072/1412059), [2](http://meta.stats.stackexchange.com/a/2559/11849)] regarding migration. In principle, your proposed guidelines should work well although I'm not sure what's new about them. 

However, what needs to be addressed are some technical issues:

Quite frequently, the following happens: (i) A (new) user asks a question that is off-topic, but would fit well on the sister site. (ii) The user gets advised that the question would fit better on the other site. (iii) The user cross-posts (sometimes he had done so anyway). (iv) The question gets migrated. Now, there are two identical questions on the sister site. (v) The cross-post needs to be closed.

Thus, you should prevent this scenario using technical measures and also mention cross-posts in your guidelines.

I've also never understood what the rules regarding comments are. Sometimes I've seen valuable comments be deleted during migration, sometimes irrelevant comments are migrated with the question. Your guidelines should address this too.