Let's talk about cities!

How many citizens could an average hive tower (spire?) hold? If a city has one, is it automatically a hive city, or does it need the layers underground as well?

I'm curious because I figured only hive cities had hive towers, until I read Sabbat Martyr. It'd make sense to build them in a city with limited space but overflowing population, I guess.

I also understand that you need to be at a young age in order to become a SM. What is the max age limit do you think?

Hive Cities aren't initially built where there is a large population, they're built and then the population is shipped in to fill them.
Hive Cities are just many layered, sprawling metropolis' that don't have to have a large underground section- a large Hive City could hold about 1 Billion people so I'd imagine a sizeable Hive Tower could hold at least a couple of million.
Well your body still needs to be developing so I imagine the latest the Astartes could reasonably leave it would be about the age of 16-17, there are precedents for adults becoming Space Marines but the success rate is like 0.1%.

When an Inquisitor is raised to rank within the Ordo Malleus, they receive a daemonhammer, which is the symbol of the Ordo. It's pretty much an epic version of a hammer and I can imagine how heavy that thing is. The question is what if you can't pick it up? If you are an Ahmazzi-esque Inquisitor and you don't walk around with power-armor. Or if frankly you don't have the arm strength to haul that thing around? Just stick it in the closet?

You've pretty much answered your own question

If the Inquisitor can't use it effectively, or it's just doesn't fit with the way he works then he (or she) won't use it and will just put it in storage.

I would kind of feel bad. You've got this weapon of pure awesomeness and you kind of feel....weak because you can't haul it around. But when you see the other Malleus Inquisitors panting and carrying the massive thing you just laugh and do a little dance.

Well, the Adepta Sororitas fit antigrav suspensors to their heavy bolters to make them less unweildy (50lb heavy bolter instead of its actual 150lb weight)... I would imagine the Inquisition has access to such suspensors, so it would be more up to the Inquisitor in question of whether he/she wants to hang it over the fireplace as decoration, or carry it into battle, based on their preferred method of combat. And I think given how the Inquisition works, nobody's going to hold a gun to the back of your head and tell you to use the hammer (except, well, maybe a more powerful Inquisitor who insists it's your duty to carry your badge of office, like all Chaplains have Crozius Arcanums... and now C:SM says your chappy is armed with one and can't swap it for nothing.)

Well, as you remember, the soul of Horus was completely and utterly obliterated by the Emperor of Mankind. However, when the Emperor "died" He was still concious in spirit, not body. Think of the Warp of a separate Universe with two (or more) sides. One side (the smaller one, by alot) being protected by the Emperor of Mankind. So in here there would be the Souls of Vulkan, Corax, Ferrus Manus and such. Sanguinius would not be in there because of "family" issues.

In the chaos side, there are the four planes of the Primordial Annihilator (title taken from Legion) basically, the chaos gods. Each god has their own domain, so basically their own planet.

In between these "areas" is (i believe, im not sure) the location of the Eldar Webway.

Is the discharge of a Melta-gun/Multi-Melta visible to the naked eye?

From the descriptions I've read, the weapon heats its target through the use of microwave energy, which I believe, is invisible. But I believe I read in the BL book Fallen Angels that when their Melta-gunner fired on the rebel APC's, it emitted a 'beam' of some sort. (I lent the book to a friend so can't verify).

There was some talk about the Aquila but how do you do the Mechanicus sign? The Cog?