That's a very generic statement on the Engel site, a 750ml tank may well be enough for many models, but others may need more, some may need less.
Generally speaking, the older boats tend to have higher freeboard, so you will very likely need a larger tank to sink these beasts.
Also how is the boat being built- wet hull or dry hull?
Does any of the watertight enclosure, be it box, cylinder whatever project above the surfaced waterline?
The way to size the ballast tank has been well documented on this site in the past. Do a little searching and you'll find some answers.
Piston tanks offer a very precise method of ballasting a submarine, however if you use a single tank, you will end up with a shifting C.G as you fill and empty the tank. The longer and slimmer the tank, the worse the problem becomes.
In the real world this isn't as big an issue as you think, as submarines tend to adopt a nose down/nose up attitude as they submerge/surface anyway.
However one way of overcoming this is to use twin tanks that work in opposite directions.
Waterpump systems work well enough, and are very cheap to build and operate. One problem I have noticed in the Sheerline models, is inadequate baffling of the ballast tank.
A fellow club member purchased a S/H Eden nee Sheerline Trafalgar class sub recently.
The boat had been constructed well enough, but he was disappointed with the handling, as it had a tendency to 'wallow' in the pitch axis.
We put this down to a half filled ballast tank 'sloshing', as the dynamic stability of such a model should be good, and it had plenty of lead in it's belly to effect a decent metacentric height.
The tank was dismantled, and one baffle found inside- inadequate IMHO for a tank that size.
The tank has been shortened (it's going in a different hull) and extra baffles inserted. Test runs are imminent.
So the moral is, if you use a ballast sytem that incoporates an exposed water surface, baffle your tank well if you want a stable boat.
Piston tanks don't have this worry- no exposed water surface.