My DD also had severe reflux, and was a grazer with feeding, so also had barely any tummy time. She didn't crawl till nine months, started pulling up at eleven months, and cruising at around a year, and has only just started properly walking at almost 17 months. She cut her first tooth at about eight and a half months.
It's all within the realms of normal, and I am not worried. I think the lack of tummy time didn't help, but it doesn't really matter in the scheme of things if they are not the first to do these things. TBH life has got a lot harder now DD is walking, as she wants to do it constantly, which is knackering, both in the sheer exertion of following her round, to the mental strain of watching her every move to ensure she doesn't maim herself.
Research shows the age at which children reach physical milestones such as crawling has no bearing on later intelligence. My HV told me they would only be worried if a child was not moving at all by a year. If the child is rolling and reaching for things they'd be happy.
I know what you mean about classes though - there's always one who walked by 9 months etc. I have let go of any expectation or desire for DD to be first with anything, and I am finding it far less annoying, as a result.