I think [citation needed] that the linux kernel picks the fastest swap of all the available ones. And you could use swap files too (that can be put in MyDocs if you want, and put a low prioriity to it).

About size of swap, could depend on what way of multitasking you are using and how the app is using memory. If i have i.e. fbreader with a book loaded in background while you are doing other things in foreground, probably won't be so damaging that it could be in a "slow" swap while the fastest one is used by really active apps.

But of course, could be physical things to consider, like doing a lot of rewriting on flash memory devices, not very sure how it works specially related to the kind that uses the N900 to say that it could be bad or good in the long run..