Yep, size of your tank is needed to recommend tankmates.
And yes, silver sharks or Bala's can grow 12"+ and need to be in shoals of 4 to 6. They are also active fish and can be jumpy so having atleast a 150G+ tank is recommened to give plenty of space
OK, I dont know how much you know on Fishless cycling, which obviously you cant do on the 6g as it has fish already in it and hopefully if given to you with the tank will already have an established filter. But I strongly recommend you have a read of this thread: http/www.fishforums.net/content/New-to-t...shless-Cycling/ as it will help you a great deal in keeping happy and healthy fish in the long term.
Whilst your tank is cycling, you could have a good read through the pinned topics on the forum about fish species, many with detailed descriptions including size and their requirements and what size tank is best for them, also how many of each species are needed for them to be happy.
Everybody here will be more than happy to help you with cycling your tank and advice on which fish you like. Read around the forum, it really is a good place and you will learn a lot here.