Even without the pump running, they will bunch up on one end due to the breeze (even a light one.)
Would not recommend placing glass floating directly into the pool. One broken candle holder and the entire pool will have to be drained to find the broken pieces. Cost would far exceed any potential profits from the risk.
You can place the candles/cups in groupings on styro sheets covered with flowers and/or foliages and then anchor the sheets via fishing line to painted bricks which match the pool bottom's color.
One word of caution: you MUST place the anchor bricks on flat spots in the pool's bottom. If the pool slopes, the bricks will 'walk' and can drag your float under water.
There is no inexpensive way to do what your customer is requesting. It takes preparation, skill and time to be done right.