Imperial Fist recruitment fluff

[thepuritan]

i realize this might not be the best forum to ask this question, but here goes...

does anyone know what the process that recruits have to go through in order to become Imperial Fists?

the reson i'm asking is, i wanted to write up a backstory for my 9th company Captain.

i wanted him and his brother to grow up training themselves to be Space Marines.
when they finally get a chance at recruitment, i wanted the future Captain to pass every entry test (despite being small for his age)... but when they perform a DNA test on him (to see if his body could handle the Geneseed implantation), they find that there is such an extremely slim chance of him surviving.
so, they deny him entrance.

almost immediately, there's an attack on the recruitment center (or ship... or whatever) where his brother (and, apparently, the guy running the diagnostic machine) die.
the future Captain takes the ID card from his brother's dying hand and vows to fight on his behalf.

in typical heroic fashion and despite the odds against it, he manages to hold his body together through the transformation.

he achieves distinction early, but his (now relative) small size keep him relegated to the leadership of the Devastator Reserve Company.

could that work within canon fluff?

[Zaphod Beeblebrox]

I'm unsure whether Imperial Fists have any particulars when recuiting, other than the fact that they recurit from several planets as they see fit, rather than a set homeworld, as the Ultramarines do. Other than that, I think they follow the standard procedures, young adolensants with potential and loyalty are screened mentally and physically before slow gene changes take place, and physco-training is done. There is a very small chance of any marine surviving the process, but they do look at people very closely and deny them if there is a chance that they have DNA weaknesses or a weak mind, so of the most part that would work.
THe biggest problem is that space marines have to complete the modification to the full before they have the balck carapace that allows them to interact with their power armour. However, scouts don't have this and are still marines, and they can have variations in size and strength due the the fagile nature of gene seed implantment, so it would seem a reasonable story to me. Go with it!