making a list seems to be quite complicated.
True. I'm slightly tempted to cut back Trustworthy to a flat 33% (as opposed to its current "50% of stuff that isn't FROM SPACE") and then Marines back to 25%, which'd simplify things moderately well.
I can go space marine heavy, in which case 33% or more points must be space marines AND a further 17% of points must be trustworthy
Well, yes, but I think it'd be easier if you thought of it as:
33% of my army must be Space Marines.
50% of the non-aerospace/spacecraft portion of my army must be trustworthy.
in both cases I cannont have more then 30% of the aerospace support, which also affects the trustyworthiness score.
Nope. Aerospace support is ignored completely. It's 50% of your non-aerospace, non-spacecraft units (so if you have 500 points of Aerospace, you need 1250 of Trustworthy).
Unless you mean that that makes things messy by fiddling with the numbers like that. In which case...yeah.
To be honest, unless you've got something clear in your head, i wouldn't bother with the space marines. I have trouble picturing them working alongside xenos, or being present in such large amounts (diplomatic bodygaurd, yes) and the best reason i can think for having them in the list is to make buying units easier in real life. But there's quite a few alien races available in 6mm now.
They're quite well-established as tagging along in the background:
First Book of the Astronomican: Tyrel Cathek. Rogue Trader. One company of the Stone Hearts chapter and two companies of Imperial Army (IIRC). That's a minimum of 1000 points - more if you buy a few upgrades or invest in some vehicular support (like the Thunderhawks to land that Battle Company).
In C:SM 3e, a Rogue Trader had a Librarian, a Captain, a Chaplain, 30 Tacticals, and 20 Assault Marines under his command (as well as several other elements not represented in Epic). Along with three Thunderhawks. That's a minimum of 1375 points.
You don't really need that many Marines to get to one-third of the army.
As to cooperating with xenos: I think it'd depend on the context, on the xenos, and on the Marines. I don't think any of them would ever be thrilled about it, but I think they could probably deal with it in certain circumstances - the rules are different on the fringe of the Imperium, and Marines are smart enough to recognize that. Sometimes.
the kroot can get an upgrade called walker, but no upgrade is then listed.
I also forgot the Terminators option. Double oops.