well i think it works better minus experience
Yeah, I definitely agree, although I do think Shirou would at least
start training them once he found out about the war.
and I think that given the growing up together I would see it as the slave thing being a formality. which makes it more palatable
Well, that was the general idea. Essentially, they're slaves only in the bedroom, and then only because they
like being treated like that. The rest of the time Shirou treats them as friends and equals.
However, since they are formally slaves, they would be treated as such by outsiders. Also, in a society like that it is inevitable that the education system would reinforce the system as it stands, because that is how systems like that sustain theirselves. So, they would be constantly reminded that their "role" in life is to pleasure Shirou and do whatever he desires throughout their teenage years. Since he is someone they love (because he's always been kind to them despite the fact that he could use them in any way he desires, and because he's generally a good and attractive guy) rather than someone they are being forced to service, they are excited by the idea, and being dominated by him becomes part of their fantasies, and something they genuinely look forward to.
However, the nature of the Grail War means that he does need to ensure they will follow orders for their own safety (since he's far better trained than they are, and they are prone to making silly mistakes), and thus he gets them to agree to do so, with the proviso that they will be punished harshly (in a sexual manner, so it's not entirely negative since they enjoy being dommed) if they disobey him. They agree fully to this and are quite OK with it, because they know he is genuinely concerned for their safety, and won't do anything to them unless they really have fucked up (and even then won't do anything to cause them real harm).
Further, he also needs to implement a rushed training program for them to get them ready in time for the war, and that means he needs to be harsher, in the same way as a Sgt. Major in Army Basic Training is harsh. Of course, since they're in a sexual relationship with strong BDSM elements, that "harshness" manifests itself in that form (although, again, he'd never do anything that caused them any real harm).
and then decide the servants summoned.
Yeah. We also need to consider how Toshi will fit into this (since he's not going to be fucking Rin or Sakura).
Most likely Shirou gets Saber, but I'm not sure about Rin or Sakura, other than that Rider will be with Shinji (until Shirou manages to capture her).