If you do not want to download shipsets, you can just leave everything as it is and let the game choose a stock shipset for those races. The problem is that the shipsets may change everytime you load a turn. What I mean is when you load a turn and one player used a shipset which you do not have on your computer, the game will choose (random?) a stock shipset for that race. Next time when you load a turn, the same race may be using a different stock shipset. You can live with that of course but... There are people putting lots of effort in making a nice-looking shipset for others' playing pleasure so I think that 100 kbytes is worth downloading.
