KarTuneMan is correct.
More importantly, though, Toyota/Scion/Lexus remotes do NOT work when the engine is running........regardless of what brand of remote start you install, when the engine is running from the remote start, the remote will not unlock the car.
But, to answer your other questions.
--The Scion remote would not be able to arm/disarm the Compustar. Code Alarm made a unit that works like that, which was sold at Circuit City, but it's not around anymore.
There are "keyless upgrade" alarms that work off the factory remotes; I install plenty of them. But they don't have remote start or any 2-way paging features.
There are remote starters that operate off the factory remotes.......but the range is limited to what you have now (not a lot), and you're still stuck with the problem of not being able to use the remote when the engine is running.
--If the Compustar really does sense your proximity to the car, and arm/disarm itself, that would be SUPER cool!
The Corvette had a system like that in the early 1990s. There's also a cheap version of it that appears on eBay from time to time, from Hong Kong.
--Dome supervision:
The manual-transmission remote start needs to "know" the door-ajar status of your car, to properly and safely put it into reservation mode.
If domelight supervision is installed, the installer will no longer be able to use the domelight wire as his door-ajar status wire.....the domelight supervision will probably cause the reservation mode to not work correctly.
The installer could use the dome supervision output to directly turn on the domelight with a relay........but use the car's individual door pinswitch wires for his door-ajar status.........this method takes a little more time and thought, but it should work.
Also, on your car, the domelight automatically comes on when unlocking, but ONLY when the engine is off.
Go ahead, see for yourself. Unlock the car by turning hte key in the door, and the domelight will come on.............start the car with one key, unlock the car with another key......the domelight will NOT come on.
Overall, if you can just live with the domelight not coming on when the car is running under remote start, you may not want to bother paying extra for the feature.
-----Your Scion keys:
You didn't really ask this question, but here it is.
The Scion remotes will still work, as long as the engine is off......but if the Compustar alarm is armed, the factory keyless will simply make the alarm go off.
Still, as long as you don't arm the Compustar, the factory remote will work fine.......so it may be handy when valet parking, lending the car to a friend, etcetera.
So, here are your basic options, depending on your budget and preferences:
1. Carry the Scion key and the Compustar remote, and remember never to use the Scion remote when the Compustar is armed.
2. Carry the valet key and the Compustar remote......but there is something that the valet key will not do......probably won't open the trunk or glove box.
3. Go to a dealer, or locksmith, and have them cut and program a black Toyota master key.