Question: ἀποκαλυφθῆναι Is the aorist and would be translated as “was revealed”

Does μέλλουσαν (present tense) “is about to be” somehow change the aorist ἀποκαλυφθῆναι to the future tense?

Can someone explain how this aorist is translated as a future tense based on the rules of Greek grammar?

Answer: 
The operative verb in Rom 8:18 is ἀποκαλυφθῆναι (= "to be revealed") which is Aorist Infinitive Passive. Thus, it has no tense at all. Such verbs are always accompanied by another verb somewhere close by to give the tense. Thus we might have:

* was to be revealed = past
* was to have been revealed = past perfect
* is to be revealed = present
* yet to be revealed = future
* etc.

[Note that English does not have an aorist infinitive and so it cannot be accurately translated except by the phrase (in this case) "to be revealed".]

In the case of Rom 8:18, we have the near-by "coming glory" which shows that Paul intends the aorist infinitive is to be understood as a future tense. Thus, it is correctly translated by modern versions.
In Greek, the future tense can be formed in several ways, one of which is by using the auxiliary verb μέλλω (mello) followed by the infinitive of the main verb.

So, when μέλλουσαν (mellousan) "is about to" is combined with the aorist infinitive ἀποκαλυφθῆναι (apokalyphthenai) "to be revealed," it creates a periphrastic construction that expresses the future tense: μέλλουσαν ἀποκαλυφθῆναι (mellousan apokalyphthenai) can be translated as "is about to be revealed."

This type of periphrastic construction is common in Greek and is used to express future actions or events. It is important to note that this periphrastic construction does not change the tense of the aorist infinitive itself, but rather creates a future tense meaning through the use of an auxiliary verb.