So I recently purchased a Maruzen M870. I've got a few questions about it though.

Mainly it's can it handle propane? I saw somewhere saying that you need to replace a hose somewhere, but it didn't say where or how exactly. I don't have duster and didn't plan on using it anyways...

Next is how do the shells hold up to abuse? The blue ones. Will they take the drops constantly without breaking?

Lastly, is there any way to extend the shell capacity? 4+1 is a little small.

Anything helps, thanks.

I was seriously considering purchasing the gun for months, so I did research. If you want sources, I'll try and find them later for you, but what I say is what I remember.
Cannot handle propane stock, the gun is designed around duster gas. Do not unless you upgrade the system (Redwolf has an upgrade they do, but I'm not sure how much they replace.) The hose can reportedly be replaced with that found for aquarium use.
Shells hold up fine, the lips are made of brass and the shell body is a plastic. The main issue is with the pot metal extractor failing due to the interaction. There are a few ways to remedy it from placing o-rings around the end of each shell to modifying the bolt.
Shell capacity has been attempted by someone on Arnies. Look up one of the reviews on it. The relevant information should be there, but from what I recall, the tube magazine for a real shotgun did not work out well. May be worth asking Redwolf Airsoft where they get theirs, as they offer a 7+1 M870. Or, just pick your shots.

There's someone I know at TAA who regularly uses shell-ejecting Maruzen gas shotguns including some M870s and M1100s. I notice that he also has put o-rings around the base of the his shells. His shells go flying all the time, as he usually doesn't use a shell catch, and I haven't seen any of them break from hitting the hard floor.

Out of the box, you probably want to avoid propane and even green gas. The same guy I have just mentioned did mod his gun's reservoir for an external air rig though, allowing him to use a CO2 tank (but he might switch to a different type later) for better air efficiency and more power.

The stuff on Arnies helped a lot, but when people say the Maruzen can't handle the propane/gg what parts will fail exactly?

And I assume the o-rings should be tight on the shells so they don't come off, right?