Yes, scats will grow larger than a 30g, and ideally need to be kept in a school. Doesn't necessarily have to be other scats, a lot of people use archers or monos and on the whole it works pretty well.
Figure 8 puffers need less salt than a scat long term, so they wouldn't make great companions for that reason. An SG of around 1.005 is perfect for figure 8 puffers, an SG of around 1.010 would be better long term for scats. There are alternative puffers, specifically tetraodon fluviatilis, the topaz puffer. That fish is often m,istaken for figure 8 puffers in stores, grows larger, and can handle the higher salinity. Check that out for a decent alternative if you get a bigger tank.