In a perfect world you're free to do whatever you like..it's all your own work after all. In reality though, that's not always the case.
First up..did you sign contracts with these labels you mention? If so, then there is probably a clause in there about releasing these tracks for a certain amount of time..and this has nothing to do with if they keep in contact with you or not during that time.
If that time period has expired, then..yeah..you can re-release them if you want, as you won't be shooting yourself in the foot by having out there on multiple releases at the same time. It's not like you're a house-hold name, where fans will buy anything and everything you put out..well, not yet anyway.
And that brings me to my last point..do you really want to "greatest hits" album already? If not, then resit the urge to use these old tracks..otherwise you run the risk of your new album being viewed as just that..a GH release.
Now, with the common sense advice done and dusted..I just want to add I think you should do whatever you feel is right for you. If it were me, I'd go back and rework those tracks..change them a little in some way, so as to freshen them up before putting them out there on my new album..that is, as long as there isn't going to be issues with past contracts or anything.
Sorry this isn't more helpful..but it might help you focus on what you will do in the end.