Im thinking 6061 yes. But the machinist will cover all that with you. He will know which ones will have the right finishes, and will do what you want. I think 7k alum is just too hard for this application.
The RC car parts we have made, are done in 6061 cheaply, and 7075 for the expensive ones. But they hit walls going 50 miles an hour. Your pistols should be able to be done with the cheapest alum.
The other question is does any of it need to be 'anodized' or is it all base color. Only part of that alum can be anodized.
Im looking at the link you posted for kropserkel, and the gun doesnt look as bad as I thought.
The barrel is probably 18 dollars worth of materials. The grip is another 30. Now, im not sure how the grips go to gether, but it appears they may be three pieces, or two pieces (both sides with a hollowed out insert).
The emitter, and rear of the gun are not that big ofa deal. The main body isnt so bad either.
The grip will be a nightmare for anyone but someone really good at cad to create. I notice theres a 3d design on the site, But again, 3d 'visual' and cad are two different animals.
The grip is almost hand chamfered. It looks like it was sculpted, and then die cast or something. Its not a perfect chamfer so thats where the most of the design work will go in on the cad drawings, and thats where you have the most room for error.
I think that you have about $50 worth of components here + the machining.
I think its doable for a blaster for 150 bucks.
Im not sure if aluminum is a 'tumble' polisher or not. But some metals are 'tumbled' in a vat of ceramic beads, and it makes it shiny/buffed. Regardless you have at least 3 fimishes in here, which is a little extra money.
If you can get past the hand grip part, your golden.
Again, im not trying to be a jerk in any way, I just dont want you to get taken advantage of.
And my client has not returned my email yet, but he will and I will ask him for a quote and to contact you at least for a second opinion.