A bigger tank has so many benefits over a smaller tank. Not only do your options for fish increase insanely, they are also easier to care for.
With a smaller tank there is little margin for error. There is a smaller amount of water which means toxins can build up faster in your tank. It's easier to make mistakes and with mistakes come fish deaths(which can get costly)
With a larger tank there is all that water volume for toxins to dissipate, less upkeep with water changes and all around easier care. The financial difference between tanks isn't a lot either really. Kits go on sale often as well, you could easily snap up a great sale if you have patience to wait. I also suggest checking out sites like craigslist. Lots of hobbyist get great deals on used tanks.
You said you have a pleco?? What kind and how long has it been in the 10gal?
As for the QT tank, it's a fabulous idea, again it can save you a lot of money. If you were to introduce a sick fish into an established aquarium it can potentially destroy your tank.
For instance, I had a 20 gal with harlequin rasbora and neons. When I purchased my neons I didn't have a QT and just put them right into my tank. I had 2 deaths within a few days, I thought for sure I had a case of NTD(neon tetras disease) It's really common and not just subject to neons. NTD will kill off most kinds of fish. There is no cure and once it's in your tank there isn't much you can do. Luckily I just had a bad batch but if it was NTD I would have lost at least 50 dollars worth of fish and would have started all over again with my entire tank,cycle and all my fish. So potentially the 50 dollars worth of fish lost, then the 50 dollars for replacement fish and there is 100 dollars that would have been easily avoided had I had a QT tank.
Just something you can pass on to your parents. Worst case option find out how much money they are willing to fork out, ask for it in cash and either A - save it put towards a new tank or B - go second hand.
I have a 20 gal Marineland tank. I really like it. It has a 150 biowheel filter which is a neat set up I've never seen before and has day time and moonlight LEDs. I managed to nab mine on sale for about 90 bucks. I live in Canada though.
