If you have 600GB of data, I presume you have a total drive size of 1TB inside. Thus, I'd allocate the whole 2TB as TM disc, if not acquire a 3TB or bigger drive for this. Just because you have 600GB data today doesn't mean it will stay that size.
MAX drive size inside multiplied by about 3 = ideal TM drive size
With a separate need for only 50GB, I suggest picking up a cheap USB drive or another small drive for that storage.
Actually, I might go the other way on that assumption about 1TB internal. I'd use that 2TB for whatever you want in that (initial) 50GB and allocate maybe $100 to buy myself a new external HDD at about 5TB for TM. I see some 3TB HDDs for about $60 if money is really tight. Either would give you plenty of TM backup space and 2TB for general purpose storage beginning with that 50GB.
Note: again, that multiply-by-3 approach is not a requirement- just a good, general recommendation. Your current 600GB of data times 3 is about 1.8TB, so what you are thinking technically works with that rule of thumb right now (about 1800GB for TM with up to about 200GB free for that 50GB of other data). However, that 600GB will inevitably grow. In only 50GB of growth, it will start eating into that rule of thumb 3X allocation. I would guess the 50GB will likely also grow in time.
As to volume vs partition, volume is usually associated with a whole drive while partition splits up a whole volume into what seems like 2 separate drives (perhaps the terminology has been evolved since the last time I learned it). In your case with that 2TB, you might partition that volume into 2 partitions: one at maybe 100GB (2X of the 50GB you have to store now) and the other at 1900GB for TM. But if me, I'd probably just allocate the entire 2GB "volume" to general storage starting with that 50GB and buy myself maybe a 5GB external HDD for $100 solely dedicated to TM purposes.