Whoa! One month away?! That's not a lot of time to make a transition for any of you. Lot to deal with for you all: separation anxiety, jealousy, exhaustion, excitement, new friends, new fun and play, new learning, and plain logistics of it all. Given that, here's my rambling two part answer with my own experiences with 2 kids and also with the Montessori system:
1. On starting preschool: Putting your daughter into preschool is a personal decision based on you and the father's feelings and ideas and also on your daughter's maturity and readiness for "school". 16.5 months is not too young for some children, but too young for others. I think if you are worried about if sending her off to school when there's a new baby will ruin her emotionally, you'll never know. Most definitely will if you present it to her as that. Won't if you treat it as if this is the next logical step and a wonderful experience, just like learning to feed herself and dress herself. I'm guessing there's a space available at the preschool in one month and thus your dilemma?
Logistically, getting 2 kids out the door at 8:10am to get my son to preschool by 8:30 am was a drag. Easy when my baby was a newborn, cause all I had to do was strap her into the infant seat, then infant seat into the Snap n'Go...she slept thru it all. It's a royal pain now that she's older and I have to get both of them to eat breakfast, potty, and dressed too! Even with the morning logistics, it is nice to have that time in the morning with my younger one for her to nurse (when she was younger), to nap in a quiet house (when she did still am nap), and to have mommy time to play at home or go to Mommy&Me type classes. My older one gets mommy time after she goes down for the night. Some told me to get a babysitter for the baby so that my older one could have mommy dates, but that just wasn't realistic for us and it just made him more jealous when baby was around. He had to learn that we all were a family together and that his sister wasn't going away. So, if we were going to the zoo or the park, then we all were going....Funny, something my now jealous younger one is learning when big brother has the day off from school.
My son started all day daycare T/Th when he was 9 months old, then he moved to a hard core full time 8:30-2:30 AMI certified Montessori school when he was 22 months. I just happened to get pregnant when he started preschool. He was a little immature but very intelligent and mostly ready for it. He's always been very "mommy-needy", needs constant interaction, and can't handle being by himself. His teachers have always said that he's very intelligent and an excellent student. Other parents have noted what a good student he is and how well he performs in class (as opposed to when Mommy is around!) I think sending my son off to full time school just when I got pregnant had very little impact on him developmentally or emotionally. He would have been needy regardless, but school probably helped him become more independent. It would have been nice to have him only go on a part-week schedule as a 2 yr old but that wasn't an option. He had separation anxiety from 11 months to 3 1/2 years old!! He screamed and cried every day with some days having to be pulled from my legs. Lest you think I'm an insensitive mom, I knew he stopped as soon as I left, and he miraculously happily left me the next day when I finally told him it was okay to cry but he'd have to do it in the "principal's office" because he was upsetting his friends.
When my daughter turns 2 yrs this spring, she will start preschool, a compromise between my husband who wanted her to start "school" at 8 months and myself who wants to wait until she's 3, at a pseudo-Montessori school 3 days/week. She's very mature and totally ready for it, and she's probably been ready for the past 6-8 months. She has always been eager to learn from anyone, is a little cautious at first but loves to explore new things. She loves to be at home and can play by herself too. She's been getting used to away time from me with a temporary sitter for the last couple of months. She's gone to daycare occasionally, mostly when we ski, and usually loves it. Dealing with her own separation anxiety was okay after a few times of sitting with her and explaining to her that I knew my "working" away from her made her sad but that she would have fun while I was gone and then I'd be back.
Do you think your daughter is ready for school? Will she be okay away from you for short periods of time (try to answer that unbiased and not has her mom)? Can she focus on things for short periods of time? Does she like to or try to do things for herself (and do you let her)? Do you think she's ready for socialization and for being around other children (the real reason for preschool after all)? Are *you* ready for her to go to school?
Either way, you won't do wrong. Put her in now and in the best case, she loves it, in the worst you pull her until later. Wait until September and the same could happen. You won't know the difference. Do what feels right and feel good about it.
2. On "Montessori" schools: Montessori philosophy allows the child to learn at his/her own pace, independently or in small groups about himself and his environment, in a comfortable child centered environment, for as long or as short as he wants to with any certain subject or material. AMI certified Montessori schools follow Dr. Montessori's research and teachings to a "T", only believe in full time (M-F) school where the children must have a 3 hour work cycle in the am (8:30-11:30) before they are allowed to run around outdoors. There is no option for going to school part time and only Montessori teachings are allowed. The teachers do not teach, rather guide the children along (hopefully, if trained to do so correctly) and present new materials when appropriate. Children are not allowed to use school materials unless they have received a presentation to do so. Montessori "Primary" classrooms are multi-aged from 2.5 yrs to 6 years, and Toddler classrooms from 1.5-3 yrs. Older children are expected to teach the younger ones how to do things.
Since Montessori is not trademarked, any school can set up and call themselves Montessori. For instance, the Montessori school my daughter will be attending uses Montessori materials but the children are segregated by age, snack, have music, circle time, etc. at specific times--all very un-Montessori philosophy. We are sending her to an uncertified Montessori school partly because we still want her to "learn" independently at her own pace, but we thought the AMI school was too hard core and partly because I don't want her going to school full time as a 2 year old. (Logistically, her school is only 1-2 min away from his future kindergarten and grammar school too.)
Bottom line about school is to make sure the school is right for you and your child, Montessori or not.
Best of luck to you and congratulations!