My default is 16 bit integer which definitely works. No, there is no way to convert whole groups of instruments - I also wanted that, and remember Translator does it. So you could use that instead, but I ran into some problems with it so just used GS editor.
Yes, that is a problem with the CPU being taxed by all these instances and you're right, trying to place each one with single instrument placement is huge in its demands. But it the only way to get the pure convolution imaging. You don't want to mix dry signal at all. So I am not even trying to get enough instances - during MIDI composing I am just using a simpler monitoring reverb, either one instance of gigapulse or an external reverb, and then freezing tracks each with a separate gigapulse instance. Not ideal for instant real-time work, but there are no real problems once you get used to how the gigapulse effects the instruments. Besides, I always want to have that final level of control of individual tracks in a dedicated, separate, purely audio mix outside of MIDI. Maybe that is redundant but I seem to love redundancy.