Flightlink baggage allowance

Hello! I am flying from Arusha to Zanzibar next month using Flightlink and am wondering if anyone has taken this flight (or any Flightlink flight) recently. I have questions about the baggage allowance. I understand we need to have soft sided luggage no heavier than 20kg (44lbs) but our tour company seems to not be concerned if the luggage is bigger than the indicated dimensions on the website (55cm x 35cm x 20cm). They said we can just pay the excess luggage fee at the airport but I'm concerned our luggage will get rejected all together? The dimensions above are essentially the size of a carry on rolly suitcase and we had planned to bring a bigger suitcase that was more of a checked bag size. Any insights would be helpful. Thank you!!