Running testosterone with Finasteride is fine. Finasteride is a 5-alpha reductase type 2 inhibitor (which you, of course, know). It is not a DHT blocker, but it does block the conversion of testosterone to DHT. Therefore, a DHT derived agent (Winstrol, Halotestin) will still function in all its wonders and will of course have all its side effects on hair, rendering Finasteride ”useless” since it is being bypassed by the DHT-agent.
People who take testosterone and are genetically prone to hair loss will get hair loss even if they take the drug Finasteride, which is only 70% effective at blocking the DHT at the receptor level.