Having a valid Eucharist is different from receiving Communion. For example, the Orthodox would have a valid Eucharist. But are we allowed to receive Communion from them? Under most normal circumstances, no. There are certain specific cases, but otherwise not. The reason is that Communion has many effects and we have to take it all into account. The unitive grace of the Sacrament brings us together as One Body, One Church. Now, receiving a valid Eucharist from a group that is not One with the Catholic Church becomes a false profession of faith as we do not really share the faith of that church where we received Communion.
Now, the SSPX claims to profess the Catholic faith, but of course there are questions about it. So its best not receive Communion from them until they are regularized in the Church. As po18guy said, the more you support them, the longer it will take for them to unite with the Church.