It really shouldn't be a problem. Many children younger than that have surgery, though the younger the child is the more risk for complications, but at 18 months, the child should be fine.
My 7mo just went to an ENT and he said he has taken out adenoids as young as 18 mo when they were huge and causing breathing problems, etc.
My 7yr old had t&a this summer -and ended 4 yrs of constant congestion and colds.
She was miserable ofr several days since eating anything hurt,and the codeine made h er feel funny but it worked. she had an eye surgery about 18 mo and no problems at all.
So it is up to you and the dr to decide if you want to do everything at once, or try one thing at a time.
My now 10 year old had his his adnoids out when he was this age and he came through with flying colors he has not ben hardly sick
good luck and I hope things work out okay