Hi, hopefully I should be able to share some of my thoughts- I did own all three lenses you mentioned above. In short I think you should keep the 50mm prime, until you replace it with something alike. The Tamron just would not do shallow depth of field & low light shooting as the 50mm primes.
The Tamron is a fine walk about lens, light and easy to handle. Whilst its not super sharp wide open, bang per buck its a capable performer. When I got this lens, I resigned the 18-70 kit from service. Personally I wouldn't class this as a portrait lens, as its too short (distortion etc), zooms are usually compromised at the extreme. As a kit lens replacement it's fab, the other choice in this range is the longer Sigma 17-70 or the cz 16-80.
Minolta 50mm 1.7- affordable prime, I would always recommend this lens if you're starting out and don't want to spend a fortune. Soft wide open, but it is an inexpensive way to get into taking nice portrait with 'blur'. My copy gave a slightly warmer tone than I wanted, the sony 50mm 1.4 is good .. but is it 4-5 times better to justify the price? Keep this one, the Tamron certainly will not replace this!
Sony 50mm 1.4- got this for a decent price, the central area of the lens is sharp wide open. Offers half a stop more light than the minolta, out of focus blur is good, suffers a bit from CA like all large apperture lens.
Maybe also consider a beercan (70-210mm f4), some say it's over rated, but it functions well as a portrait lens at the lengths 70-135. Being a zoom it's more flexible than the 50mm, has very nice bokeh/blur and affordable - you'll find a lot going on ebay for around £150. cons are:- soft wide open, suffers from a lot of CAs, and its heavier than your 55-200.