Daniel,
As long as you are not exclusive to a site or the photo is not exclusive to a site, it can be uploaded to any site that doesn't require exclusivity (which is probably the case in every microstock site).
And of course it is a good idea to upload to all of them. They often have different buyers. An image rejected in one may be accepted in others (one of my best-sellers was rejected twice in IS). Some type of image may sell better in one site than in another. In some sites, if your image doesn't sell well soon after uploaded, it may get lost in the end of search results, even if it is a good photo; then if you are lucky it may sell well somewhere else.
Regards,
Adelaide