ok... I understand the concept of " lighter is better" but here is some Battery questions !! What does the MaH mean?? is it more power and longer flight time ??? the higher the Number ??.. i realize it add's weight cuz they are heavier... But on bigger Builds " Alien X Mode 450 / 550 can i run two batteries in parallel ??? to runs GPS, Lights Etc. to get longer flight time..
or is it better to run them separated....

So when you see ESC's saying they can handle 30 Amps, mAh stands for 'mili Amp hours", so a 3000mAh battery is capable of providing 3 amps of current for 1 hour, or 30 amps of current for 6 mins.

So much of this hobby requires balancing, the more mAh of current your battery stores, the longer it will be able to produce a set amount of current, however as the mAh increases, so does the weight, until the battery becomes too heavy and you get flight times with decreasing returns.

You have probably noticed that there are 1S, 2S, 3S etc batteries, these are cells in Series, to increase the voltage, you do also get, for example, a 3S 2P 10,000mAh battery, which would have 2 x 3S (11.1v) 5,000mAh packs in parallel. These are more commonly used for planes or big rigs, as you say 450 - 500mm chassis. No reason you can't make up a parallel battery connector, just consider the impact the weight will have on performance. Your quad should produce at least double the thrust in grams, to the AUW (all up weight, including battery) of your craft.

Tom forgot to talk about C rating. You will see a C rating number or range for a lipo. Example 50C or 45-75C.
C rating tells you how fast a lipo can discharge. Because mini quads requires a lot of current to power the motors. The higher the C rating, the faster your quad gets up to speed and likely able to maintain it speed. Think of it like a bucket of water. Lower C rating is like a small hole you drill into the bucket to drain the water. Higher C rating is like pouring the water out of the top of the bucket.

The capacity determined the value of 1C. 1000mah = 1A = 1C
2500mah = 2.5A = 1C 5000mah = 5A = 1C.

You want to supply your quad with enough current.
MAX current = lipo capacity x C rating
Ex: 1500mah x 20C = 30A which isn't enough for a powerful quad. But
1500mah x 100C = 150A is plenty of power.

I hope this article helps?
https://oscarliang.com/lipo-battery-guide/