Your BMT performances will determine where you go after bmt, but
the sispec route is more likely than the Guards route.
For normal Guards infantrymen, some of them already have their BMT
in tekong before their new posting. Some of them to fill the gaps
caused by OOC ppl from the unit's own bmt. Selection is done
through an interview at tekong. The recruits are chosen for the
interview, but you can voice out if you really want to join
Guards.
If you take the Sispec route, you can still end up in Guards unit
but as section commanders for the majority. Guards conversion is
very different for men and commanders btw.
Injuries aside, i think joining Guards is not bad. Even though
fitness(IPPT) standard is higher, you get to do rappelling. I went
from failing IPPT in BMT and couldnt swim to getting Guards silver
and learning how to swim breaststroke properly. These are what i
treasure most from my NS.
Do check out the BMT FAQs b4 enlistment, and also the Guards/Sispec
FAQ if you are really really interested.