In a small studio the dumbbell range usually decides how much floor is left for everything else. The choice between fixed pairs and adjustables is really a choice about space, supervision and maintenance.
Fixed pairs: predictable, but hungry
A 2.5 to 30 kg range in 2.5 kg steps is 24 dumbbells and roughly three metres of rack. Members find the weight they want instantly and staff can see what is missing at a glance. The cost is floor space and capital tied up in the heavy end few members use.
Adjustables: compact, with caveats
A pair of adjustable dumbbells can replace a whole rack, which suits personal training studios and home gyms. Check the adjustment mechanism under load, the maximum weight, and whether the handle profile works for both presses and curls. In commercial settings, adjustables also need a rule about re-racking or the mechanism takes abuse.
Racks and flow
Two-tier racks save space but the lower tier is where dumbbells get dropped. Three-tier racks read better on a floor plan but need clearance behind them. Leave at least 1.2 m between the rack face and the nearest bench.
Specifying the set
Confirm the weight progression, head material (urethane, rubber or neoprene), handle finish, and whether the rack ships in the same carton. Those details change both the landed cost and the day-one experience on the floor.