Yes, the libdo suppressive effects of Deca, plus the DHT inhibition of propecia could be an issue. Might be wise to avoid concurrent use -- although your test dose is reasonable (depending on the concentration) and you havn't had any problems so far?????
i don't remeber the half life of Propecia / finesteride, wouldnt be hard to find with a google seach though.
Your best hair loss prevention will probably come from an AI. The good Dr Houser proposes that hair loss is simular to that of prostate growth. The interaction between SHBG, e2 and IGF-1 on the membrane surface creates more DHT receptors allowing it to do its 'damage'. And i'd have to say im inclined to agree with him.
Would explain why the rate of hair loss increases with age; increasing e2, decreasing test and as a result serum DHT (not sure about the scalp).
So keeping estrogen in check might be more effective for preventing hair loss, at the same time not comprimising your gains (as much) or increasing your chances of gyno etc.
I would personally use Nizoral,and an AI.