Simple Instrument Tracks are specifically for non-multi-timbral synths and samplers. In order to be simple, it has to make some assumptions, e.g. one MIDI channel and one stereo output. It's just not a good fit for people who use multi-timbral virtual instruments.
When you insert the synth/sampler, un-check the Simple Instrument Track option. It's not always possible to know in advance how many of the synth's outputs you'll end up using, but that's OK. You can just choose to create a single audio track initially and add more later if necessary.
In terms of reduced memory usage, the benefit is actually small. With a sampler, most of the RAM taken up is for samples, not the player. It can actually force
greater memory usage because you lose the ability to freeze each voice separately. I typically combine instruments that are likely to be frozen or bounced together, such as strings, brass and percussion, and use separate instances for stuff I'm likely to work on for a while, such as lead instruments.
If RAM conservation is important to you, you'll get a better return by investigating what memory-management features your instruments provide. Most of them have at least a couple options for reducing memory usage, such as specifying the pre-load buffer size or purging unused samples. Superior Drummer gives you a choice between 16- and 24-bit samples, with the former option resulting in a dramatic reduction in memory requirements.