What to expect from SPQR as non-Roman?

I've been playing as Carthage on vanilla Rome and I've just set foot into southern Italy. I've taken Croton and am about to finish the Brutii by taking Tarrentum as well but I'm a bit fearful of the Senate. From what I've seen my army could wipe the scattered troops of the Scipii and Julii but clearly couldn't match the Senate army so I'm a bit weary of going north with my conquest.

What can I expect from the Senate? Will they attack me if I take Capua, Arretium and Ariminum or any of these. Or will they only react if I step into their region? Also, has anyone ever seen SPQR capturing a settlement? Oh, and if I weed out all the roman factions but the Senate, will Rome still reach huge city size and trigger the reforms? I'd like to leave it last and only conquer it immediately after the Marianus reforms are triggered so I can have armored generals.

It usually stays within the region borders, but the SPQR has built-in gold injectors so it never goes bankrupt. It might decide to step out and attack your stacks if they veer to close. It can trigger the Marian reforms but if you want them as quick as possible, give them Carthage. The SPQR's settlements never rebel, and Carthage is the fastest growing city on the map. Then, after they kick in, just retake it. I don't get how the Roman reforms apply to your faction's general unit however.

You are right, the generals upgrade does not depend on the Marianus reforms, it is given by the imperial palace. I don't know how I came up with that idea considering I've actually seen the upgrade bonus in the buildings file. The idea with Carthage is nice but I guess there's no point in triggering the reforms unless you're Roman. So much for "saving" Rome for later I guess. I'll stay carefully away from them until I can get an army to match their own and conquer the rest of the Romans in the meantime. Thanks for the info.

Several things I've noticed (but not empirically tested):

The SPQR will venture outside its home province to attack invading armies or brigands, bit will not lay siege to a city. I'm sure this is due to a coded parameter that in a non-modded game, the SPQR can have only one province...Rome.

That said, if you lay siege to the Scipii capital of Capua, or the Julii capital of Arretium, the SPQR will send an army in an attempt to break that siege. Be prepared. In the case of the Julii cities of Ariminum or Segesta, or either Brutii city in the "boot", the SPQR will not get involved.

And as far as I can tell, the SPQR cannot trigger the Marius Event. I have had several campaigns whereby upon reaching Rome I found the population to be over 24k with the required Imperial Palace built, and no Uncle Marius.....