So...
New parts:
Be creative - don't copy
Nobody here minds if his work is used in other mods, or used as base for something else. Code snippets, units, improvements from other users might serve as base for other custom things, advanced algorithms, unit reskins, etc.
But please be creative and put some work on it your own. If you only change a slight detail of somebody else's work, then it's a bit too much to call it your own. So please be creative and make the derivate work to something really distinct. The person who did the original work will be happy if his work could help to create another assets, it will make the community happy, and it will improve your skills.
Modmods
Some mods get so big and have so many fans that some decide to mod it further. This is again in the spirit of this community. But please be cautious: A mod is big work. The original modder put lots of time, thoughts and heartblood into his masterpiece. Some people might have a specific idea about what they want to do, what they want to create. And some might not like it if you turn it into something else. So please ask before you modmod. It's the only instance in this forum where permission is really required. It's a matter of heart, please respect if somebody says "No".
And also here "be creative - don't copy" applies. If you only tweaked some values, then it's not really a different mod. Nobody will really play it if you only changed the strength values of some units. So again, put enough work into it to make it distinct from the original mod. It will certainly attract more players if you can put your differences into some crisp sentences, and if these then do not consist out of negligible things, then you have achieved it: A modmod.
(please note: If somebody is not anymore active, and you do not get a response for your permission request, then consider it granted; this also applies only to full mods and everything contained in them, for everything else the point "no permission required" applies)
It has also been the tradition in this forum to describe a modmod as what it is, a modmod, and to attribute the main work to the original author. One part is sure that you honour who did most of the base work. But it's also an advertisement feature for yourself. Because if you make a modmod, then the people who are most likely to try it out are the players of the original mod. If you say "modmod of X", then they'll see that it's something they should check out. And it's also interesting to see for the other people around. If you see that someone has created a modmod, then it means the original mod is good. Really good. So more people are likely to check that mod out, and so also your work. So please honour the original creator, you'll both benefit from it.
Changed parts (in both the second paragraph):
No permission required
Requiring permission to use something here on the forums is not workable. This forum has existed for more than 10 years. It will probably also exist in 10 years or more. You don't know if you'll be here in 10 years, but what you've uploaded will still be here. If you left the forum, who should be asked? If you see something uploaded in 2002, 2005, 2008, and the modder has left, who should you ask? Working with permissions is therefore not realistic. We therefore consider that you imply with your upload that what you've uploaded is free to use. Else...what would be the point of an upload?
Please note that some people put restrictions on their work in the form of licenses, e.g. the "creative commons" license (not very common in this forum, but happens). In most cases this means you can use their work, but you have to credit them and to put derivative work under the same license. Please respect that. It's at the end free to use, so please show the courtesy to respect their wish.
Give credit if you use something
Since no permissions are required, there is the need to honour someone's hard work. This is done with a "credit list", or "credits", which means that you name the persons whose work you used in your mod. A simple list at the end of your mod will do it. If you don't know who did something specific, then credit the mod team with a list of their members (Mod XY (user A, user B and others)), ... and a generic thank you to the all many unknown modders won't go amiss either.
And it would also be great if you gave some feedback at this point! Everyone likes to get some comments on their work. And who can judge about the work better than the people who dealt with it? So please try to leave a comment in the threads of the included things. You see that somebody released a unit, which you want to use? Then please be so nice and tell that the author! A "Niiiiice! I'm totally going to use that one

." will show the author that his work is appreciated and will only need a second for you. So please comment on other people's work.
--------------------
Did I forget anything?
Oh, and grammar nazism is explicitely allowed.