Using Amp 12V Source for Alarm/Starter?

I'm installing a alarm/starter on my Toyota Corolla 1998 LE, I have to bring in the 12V constant directly from my battery, since I plan to attach a window open module, and possibly neon lights and light source for the glove compartment as well. Since everything is already opened up, I also decided to put in the rough-in wiring for future speaker upgrades, and likely installation of an mono-amp for a sub. So here's the question, would it be ok to take in the power for the alarm/starter from the 12V constant wiring that I'm running to the trunk for the sub amp ?

In your connections, how did you connect  the alarm system without using the power distribution/termination blocks near the dash. Most ot the power distribution blocks are 4g to multiple 8g, and are meant for the terminating point. Whereas, i just need a T connection for the alarm system, with the 4g power wire going from the battery to the trunk. Plus, the since the distribution blocks are mostly 8g outputs for a 4g input, that's way too big for my alarms 12g power wire. What I really need is a distribution block with 4g or 8g input, and providing 10g to 16g outputs, for the little power suckets like alarms, aux lights, or leds. Can't see it being easy or clean to connect/solder the 12g to a 4g or 8g. Any ideas here ?