I definitely wouldn't risk Inhibit E. Formestane works well with Trest as long as the dose isn't too high. If you're going 100 mg or higher your best option is Aromasin.
Edit: Reread your post and saw that you asked specifically about 50 mg Trest daily. In that case I would comfortably choose Formestane, unless of course you have a track record of getting high estrogen sides. I've seen logs where guys have used OTC AI's like Arimastane for Trest but I would never risk it personally.