I've mentioned on several occasions lately that the future subjunctive isn't used in normal conversation by anyone. Do not use it.
Estuvieran and estuviesen are identical in meaning. There are two ways to use the imperfect subjunctive. The latter is used more in Spain and in literary works.
I didn't suggest that you use the imperfect subjunctive. I said that either the present or the imperfect subjunctive could be used. What I didn't say, because I assumed you may already know it, was that it depends on perceived time. If you're thinking the action is in the present or the future, use the present subjunctive
estén.
Here is a very informative
link about the subjunctive.