1) Is it advisable to start HRT soon due to my age? (i'm 26.5)
By all means. I started at 54 and I have some good development
2) How do I deal with the fertility issue? (I would like to have kids, but I know HRT makes you sterile)
As others have said, sperm bank
3) How would I approach the issue of HRT with my employer?
You do not have to tell them anything. Your records are protected by HIPPA.
4) Can I take HRT for say 1 year without them noticing and still present as male at work?
In most cases , yes you can. But you may not wish to after a point.