Hi! My question is already put in the title, but I'll elaborate. I plan having one of my Space Marine characters be a veteran, that would be Dark Angels Company Veterans Squad fighter.

Even those guys don't get both boltgun and powersword in one loadout on the tabletop, but on the other hand:
1. this makes some sense, especially for Dark Angels, seeing how a Power Sword would be a direct improvement on the Ceremonial Sword, and they carry that in combat at times;
2. I thi-ink people would want it for the cool-ness;
3. you can build that loadout in the Rogue Trader right now (given the Trader isn't nowhere near the final version, I presume).

Yet again, if this is implemented, how could it affect the gameplay? Will equipping the power sword, for example, cost more/take more space for tacticals than for the assault marines, to make sure tacts don't have enough goodies left to overpower both a dedicated melee fighter and a sniper?

Hmm.
The game wont work like the TT, you cant go straight into the game as a veteran.
All non-basic classes will cost RP to use, they can be customized to a degree.
Basic classes for space marines (all chapters) are, Assault, tactical, devestator, and an apothecary (Other factions may vary)
I think it was Midguel who said that you would become a veteran, when you reach a certain point in the game with one of your characters, by upgrading yourself in a skill tree.

As for using swords as tacticals/basic classes. There is still disscussion about being able to use stronger melee weapons as a range based class, however, there will be some melee weapons that can be used (Like a chainsword) at the cost of 'Points' like in TT. The points (as Steven explained in one of his videos) will be used for weapons/alike, so you have (in alpha) 2k points, say a bolter costs 600pts, has 10 damage-10range-10accuracy, then there is a powersword (very good in melee) it costs 700pts, has 20damage(melee,canbeblocked,mediumcleave).

You can choose between primary, secondary and melee w/ pistol (Inalpha), at the cost of the points, so you can never have an OP build, but you can have it set out so you can go for your kind of playstyle.

If you wish to use a powersword as a tactical, you can, but put some thought into what you will use for a secondary/primary, you may not have enough points for anything useful.
Hope that helps.

I believe they didn't say that you wouldn't be able to equip power swords unless you have a certain amount of experience, and it only makes sense for vets to have them (let alone the "Champion Shoulder Pad" item). And I love power swords, I want one. The exp requirememnt would make sense, but if it's not implemented I could just be roleplaying the vet, and hopefully get the exp quickly enough to support the fantasy

That's exactly what I envisioned though, thank you. It has already been discussed on the forums, hasn't it? You seem to have great insight ^^

Yes, they haven't really explained the skill tree system in detail.
From what I know, you can spend points to unlock upgrades for your weapon, and you can make a path along the tree that leads you to unlocking another weapon OR lowering the amount of C(currency) it costs to purchase it.
I'd like to hear more about how it works, I kinda hope it works the same way it does in battlefield, but you have to spend currency to unlock the weapons, so you dont have to deal with a gun sitting in your vault, taking up space.
If you haven't seen Stevens videos I suggest you check them out, he explains some things about it in one of his videos.

as far as i am aware loadouts will work like this in game...

when you go to spawn you will be picking out a loadout, the gear options available to you will depend on what class you pick to spawn as. basic classes will not cost RP (recourse points) to spawn, Elite classes will cost RP to spawn and heroes will cost RP to spawn and/or start a long cooldown.

each loadout has a pool of points (LP) to spend on gear, each piece of gear will have an LP cost to it so you may have a super powerful bolter in you loadout but it will limit what other gear you can afford as the bolter is so expensive. on a side note veterans are unlocked through progression and allow you to spawn with a bigger LP pool for the basic classes at the cost of spending RP to spawn.

with this system it may be possible to have a melee weapon like a powersword and a bolter but i wouldn't expect you to have the points left to focus your character to be as powerful as a more focused loadout. also i would expect your accuracy and melee power to suffer some if dual wielded.

also the rouge trader store is just a way of unlocking items for in game use, and they will be globally applied to your account. the loadout system in game will be different, see one of the live streams for the devs showing a mockup of how it will look.