I honestly don't see Benji Marshall as fullback. I see him as an outside centre or right wing. He hasn't got the skills like Folau or Dagg to play at fullback.
It will be very interesting to see how Marshall's transition to Rugby Union goes - personally I don't have overly high hopes for him, but I will be happy to be proven wrong! Kirwan has already stated he is looking to use Marshall mainly at fullback at the start of the season, as it is arguably an easy role to learn than 10. I suspect he may come off the bench and play fullback early in the season, with Piutau shifting to wing/centre (Kirwan seems quite keen to see Piutau at centre). With his skill-set I think 10 is obviously where he should be playing in the future - it may just take some time for him to learn the role. I assume when you state he "hasn't got the skills of Folau or Dagg" you are referring to high-ball skills? I don't disagree, however there are few sides in Super Rugby that regularly use the high ball as an attacking weapon, so I don't think this would be a major issue for him. I think a bigger issue for him will be his positional awareness at the back, as teams will strong tactical kickers could expose his naivety in this area.
Personally I can't see him being overly effective on the wing or at centre. One of his biggest strengths is his ability to create gaps for others - this would be wasted on the wing. I don't think he has the pace to play on the wing either. His creativity could possibly be used at centre, but his lack of defense skills would be a real liability here. He is known as a bit of a weak defender in league, and 13 is the hardest position on the field to defend - it would be a recipe for disaster.....
With Kaino back I see Luatua being used more in number 8 as back up to Saili or in the locks. I'm very excited to watch the Blues this year. Kaino and Marshall are the biggest draw cards for me... Phenomenal. I also think Nonu would do well now that he's played in the All Blacks with a lot of the back line.
Unless they are injured (or rested) I expect to see Kaino and Luatua start every game for the Blues in the 6 and 8 jersey. Saili is an ok Super Rugby player, but I think he will struggle to make the Blues bench if everyone is fit. I think the most likely combination will be Luatua at 6 and Kaino at 8 (as IRISH RUGBY MAN suggested). There is the option of Luatua at 8 and Kaino at 6, but personally I think Kaino is the better suited of the two at 8 (and has far more experience there). Luatua can cover lock, but I would be disappointed if they ended up playing him there, as though I'm sure he would do a good job he is far more effective as a loose-forward.
Personally I expect to see a Blues side that looks something like this at the start of the season:
1. Woodcock
2. Mealamu
3. Faumuina
4. Donnelly
5. Moli
6. Luatua
7. Braid
8. Kaino
9. Weepu
10. Kerr/Noakes
11. Moala
12. Nonu
13. Saili
14. Halai
15. Piutau
16. Parsons
17. Ta'avao
18. Barrett
19. O'Connor
20. Gibson-Park
21. Willison
22. Marshall
The biggest weakness in that side is the locks. Moli has never stepped up to Super Rugby level, and Tuipolotu could put a lot of pressure on him (if he gets a chance) if he doesn't show some big improvements from previous seasons. The Blues may even be tempted to start Kane Barrett (or Luatua) at lock, and it wouldn't surprise me if they had Barrett covering lock (and 6) from the bench. First-five is an interesting situation - Noakes did a good enough job last season (barring his poor defense), but Kerr looks to be potentially the better option at this level. Personally I would pick Kerr and stick with him, but they may well persist with Noakes (in which case Kerr could well be a bench option instead of Willison/Marshall). The back-up halfback will be an interesting choice. Gibson-Park and Hall are both talented young halfbacks with very contrasting styles. Hall has far more control to his game and has an excellent kicking game, but Gibson-Park may get the nod as his running game can be very effective against tiring defenses (and he offers a contrast to Weepu).