1. A number of the Deluxe Scenarios (e.g.. Ilipa) combine units from different legions. Should I assume that all such units are the same 'colour' and 'unit class' when playing the scenario?
No, Legion restrictions must be observed
2. Ilipa: Should line 6 of the Roman Line Command Eligibility chart include the Tarentine LCs?
BD Note: It seems obvious that the PrftEqts must be able to command the Tarentine LCs individually as well.
3. Ilipa: Can Tribunes command the allied centre units?
No, that's Culchas' job
If the Iberians can only be commanded by Culchas (Ilipa) then the deployment of Antonius is rather odd.
I wouldn't want Antonius to be unemployed - Add the following to the Roman Command Special rule: Treat the Iberian SK as Velites for the purposes of 4.42
4. Where a Roman Army includes non-Roman Leaders, is the default rule that they can't command Roman units?
The scenario should explicitly state (Ilipa - "Culchas may command only the Iberian units.") who commands what outside the standard rules. If in doubt, you can't go wrong with non-Roman leaders can't command Roman units.
5. I assume Scipio can command any unit. Does the same apply to any other 'named' Roman Leader (e.g. Silanus)?
Unless there is a command restriction on the counter per 4.28, or special rule as in 4.4, or a restriction listed in the scenario, a leader can command any friendly unit.
6. 7.25C and 10.51 talk about units that are in a Leader's command. Considering Tribunes, does this apply to any units (within Command Range) of just one Legion and class (for that Activation) or to units of any Legion and class?
Any units he could command. The Legion/Class restrictions apply