Realisitc use of auxiliaries

[Tiberius Nero]

Three related questions:

I do it on the assumption that this way they're less likely to join their brethren and turn against me (as if that could happen in RTW), but I guess that's not really realistic, is it now?

Not really, you probably give those people a chance to fight against their enemies by recruiting them in your armies.

2. Would it be realistic to build an auxiliary-only army, a full stack, even? Ignoring issues of tactical viability, are there accounts of generals leading armies composed almost exlusively of their 'enemies'?

Not for Rome, unless you say this is the army of a client state/kingdom and you restrict its movement in its particular province of origin. You could still take bits of this army to swell your ranks, but an auxiliary army acting independently, no.

3. Following on that idea, would it be plausible to construct an auxilia-only full legion in post-Augustan times? Rome used auxiliaries aplenty, organised into cohorts. Did they create auxilia-only legions, even ad hoc ones? Obviously I'm talking about times preceding the third century AD.

Not afaik; "auxilia" means "help", "subsidiaries", their very name suggests they are used to fill in gaps in a main army, not act on their own.