My wedding theme is cherry blossoms with a main color pallet of light pink, white, and grey. But I’m not entirely sure what other flowers to have with the cherry blossoms. It will most likely be in late March. (I would love roses and or carnations too but I don’t know if I would be able to get them before May). Maybe lillies or magnolias, what do you think?
I would ask your florist what's in season in your area. That will greatly affect the price and availability of flowers. You'll save yourself so much aggravation if you find out what's reasonable before getting your heart set on any particular thing. Where I live, cherry blossoms don't come out until May, but every place is different.
I think the more freedom you give your florist, the more happy you'll be with the results. Your florist will know what is in season, what looks good together, and what is durable/long lasting enough to work with your venue.
The florist should be able to provide you with a "look book" of ideas that you can pick from. So I say tell the florist you want cherry blossoms with accent flowers in your color palette, and see what they suggest.
If you do more traditional small bouquets, roses would look beautiful!! Or peonies would be a great choice. Talk to a local florist for ideas and to find out what's in season. Here's a few inspiration photos I found too.